[clang-tools-extra] [clang-doc] fix flaky test in clang-doc (PR #101387)

[Paul Kirth via cfe-commits]

ilovepi wrote:

In the description, its probably better to use the `#<PR number>` notation than linkify `PR`. Most of us will only see the raw text in the commit message, and while a full link is appreciated, it would be good to know the PR number at a glance.

https://github.com/llvm/llvm-project/pull/101387