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

[Michael Park via cfe-commits]

mpark wrote:

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

Haha yep. I suspect that as well.

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