[clang-tools-extra] [clang-tidy][doc] align the title style in clang-tidy/index.rst (PR #119938)

[Nicolas van Kempen via cfe-commits]

https://github.com/nicovank approved this pull request.

Ctrl+F -ing for lowercase `clang-tidy` reveals a couple more places where it should maybe be updated: lines 331, 350 and 352 (`` :program:`clang-tidy` ``?). It's very minor anyway.

https://github.com/llvm/llvm-project/pull/119938