[clang-tools-extra] [clang-tidy][NFC] Fix error in example code for `modernize-use-scoped-lock`'s doc (PR #159722)

[LLVM Continuous Integration via cfe-commits]

llvm-ci wrote:

LLVM Buildbot has detected a new failure on builder `clang-aarch64-quick` running on `linaro-clang-aarch64-quick` while building `clang-tools-extra` at step 5 "ninja check 1".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/65/builds/22861

<details>
<summary>Here is the relevant piece of the build log for the reference</summary>

```
Step 5 (ninja check 1) failure: stage 1 checked (failure)
******************** TEST 'Clangd Unit Tests :: ./ClangdTests/241/332' FAILED ********************
Script(shard):
--
GTEST_OUTPUT=json:/home/tcwg-buildbot/worker/clang-aarch64-quick/stage1/tools/clang/tools/extra/clangd/unittests/./ClangdTests-Clangd Unit Tests-1114149-241-332.json GTEST_SHUFFLE=0 GTEST_TOTAL_SHARDS=332 GTEST_SHARD_INDEX=241 /home/tcwg-buildbot/worker/clang-aarch64-quick/stage1/tools/clang/tools/extra/clangd/unittests/./ClangdTests
--

Note: This is test shard 242 of 332.
[==========] Running 4 tests from 4 test suites.
[----------] Global test environment set-up.
[----------] 1 test from SignatureHelp
[ RUN      ] SignatureHelp.TemplateArguments
Built preamble of size 707616 for file /clangd-test/TestTU.cpp version null in 0.08 seconds
Built preamble of size 707616 for file /clangd-test/TestTU.cpp version null in 0.07 seconds
[       OK ] SignatureHelp.TemplateArguments (213 ms)
[----------] 1 test from SignatureHelp (213 ms total)

[----------] 1 test from FormatIncremental
[ RUN      ] FormatIncremental.FormatBrace
[       OK ] FormatIncremental.FormatBrace (2 ms)
[----------] 1 test from FormatIncremental (2 ms total)

[----------] 1 test from RenameTest
[ RUN      ] RenameTest.PrepareRename
ASTWorker building file /clangd-test/foo.h version null with command 
[/clangd-test]
clang -ffreestanding /clangd-test/foo.h
Driver produced command: cc1 -cc1 -triple aarch64-unknown-linux-gnu -fsyntax-only -disable-free -clear-ast-before-backend -main-file-name foo.h -mrelocation-model pic -pic-level 2 -pic-is-pie -mframe-pointer=non-leaf -fmath-errno -ffp-contract=on -fno-rounding-math -mconstructor-aliases -ffreestanding -enable-tlsdesc -target-cpu generic -target-feature +v8a -target-feature +fp-armv8 -target-feature +neon -target-abi aapcs -debugger-tuning=gdb -fdebug-compilation-dir=/clangd-test -fcoverage-compilation-dir=/clangd-test -resource-dir lib/clang/22 -internal-isystem lib/clang/22/include -internal-isystem /usr/local/include -internal-externc-isystem /include -internal-externc-isystem /usr/include -ferror-limit 19 -fno-signed-char -fgnuc-version=4.2.1 -fskip-odr-check-in-gmf -no-round-trip-args -target-feature -fmv -faddrsig -x c-header /clangd-test/foo.h
Building first preamble for /clangd-test/foo.h version null
Built preamble of size 730268 for file /clangd-test/foo.h version null in 0.19 seconds
indexed preamble AST for /clangd-test/foo.h version null:
  symbol slab: 0 symbols, 120 bytes
  ref slab: 0 symbols, 0 refs, 128 bytes
  relations slab: 0 relations, 24 bytes
indexed file AST for /clangd-test/foo.h version null:
  symbol slab: 1 symbols, 4448 bytes
  ref slab: 1 symbols, 1 refs, 4248 bytes
  relations slab: 0 relations, 24 bytes
Build dynamic index for main-file symbols with estimated memory usage of 11520 bytes
ASTWorker building file /clangd-test/foo.cc version null with command 
[/clangd-test]
clang -ffreestanding /clangd-test/foo.cc
Driver produced command: cc1 -cc1 -triple aarch64-unknown-linux-gnu -fsyntax-only -disable-free -clear-ast-before-backend -main-file-name foo.cc -mrelocation-model pic -pic-level 2 -pic-is-pie -mframe-pointer=non-leaf -fmath-errno -ffp-contract=on -fno-rounding-math -mconstructor-aliases -ffreestanding -enable-tlsdesc -target-cpu generic -target-feature +v8a -target-feature +fp-armv8 -target-feature +neon -target-abi aapcs -debugger-tuning=gdb -fdebug-compilation-dir=/clangd-test -fcoverage-compilation-dir=/clangd-test -resource-dir lib/clang/22 -internal-isystem lib/clang/22/include -internal-isystem /usr/local/include -internal-externc-isystem /include -internal-externc-isystem /usr/include -fdeprecated-macro -ferror-limit 19 -fno-signed-char -fgnuc-version=4.2.1 -fskip-odr-check-in-gmf -fcxx-exceptions -fexceptions -no-round-trip-args -target-feature -fmv -faddrsig -D__GCC_HAVE_DWARF2_CFI_ASM=1 -x c++ /clangd-test/foo.cc
Building first preamble for /clangd-test/foo.cc version null
not idle after addDocument
UNREACHABLE executed at ../llvm/clang-tools-extra/clangd/unittests/SyncAPI.cpp:22!
Built preamble of size 739968 for file /clangd-test/foo.cc version null in 9.57 seconds
indexed preamble AST for /clangd-test/foo.cc version null:
  symbol slab: 1 symbols, 4448 bytes
  ref slab: 0 symbols, 0 refs, 128 bytes
  relations slab: 0 relations, 24 bytes
...

```

</details>

https://github.com/llvm/llvm-project/pull/159722