[clang] [PAC][Headers] Fix incorrect comment in ptrauth.h (PR #207265)

[Anatoly Trosinenko via cfe-commits]

atrosinenko wrote:

Changed the "C++ v-table pointers" wording to "pointers to C++ v-tables" along the way, as the former looks slightly ambiguous.

https://github.com/llvm/llvm-project/pull/207265