[clang-tidy][docs] Fix wrong sphinx link after d9afb8c3

There was a copy-paste mistake at the embedded link:
  `clang-tidy/checks/cppcoreguidelines-virtual-class-destructor`
  ->
  `clang-tidy/checks/cppcoreguidelines/virtual-class-destructor`

Sphinx error:
/home/zbebnal/git/llvm-project/clang-tools-extra/docs/ReleaseNotes.rst:168:unknown document: clang-tidy/checks/cppcoreguidelines-virtual-class-destructor

Build bot: https://lab.llvm.org/buildbot#builders/115/builds/29805

Differential Revision: https://reviews.llvm.org/D126891