[NFC] [Doc] Fix example for AnnotateTypeDocs

Change clang::annotate into clang::annotate_type.

The example will get error like
error: 'annotate' attribute cannot be applied to types
int* [[clang::annotate("category1", "foo", 1)]] f(int[[clang::annotate("category2")]] *);

Reviewed By: aaron.ballman

Differential Revision: https://reviews.llvm.org/D139935