[clang] [clang] Fix Clang language extension documentation markup for __builtin_arm_trap. (PR #85310)

[Tom Honermann via cfe-commits]

tahonermann wrote:

@TNorthover, doc builds are failing because of this.
```
Warning, treated as error:
/home/runner/work/llvm-project/llvm-project/clang-build/tools/clang/docs/LanguageExtensions.rst:3447:Title underline too short.

``__builtin_arm_trap``
------------------
```

https://github.com/llvm/llvm-project/pull/85310