[clang] Fix a typo in InternalsManual: ActOnCXX -> ActOnXXX (PR #105207)

[via cfe-commits]

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

LGTM

We do have some `ActOnCXX...` functions, so that’s probably how that happened ;Þ

https://github.com/llvm/llvm-project/pull/105207