[clang-tools-extra] [clang-tidy] Proofread Contributing.rst (PR #149831)

[Baranov Victor via cfe-commits]

vbvictor wrote:

I'm not sure on the `false positive` -> `false-positive` change.
We typically use `false positive`, you could grep docs or look at [release notes](https://github.com/llvm/llvm-project/blob/release/21.x/clang-tools-extra/docs/ReleaseNotes.rst).

https://github.com/llvm/llvm-project/pull/149831