[clang-tidy] fix link in Release Notes

Summary: This is intended to fix https://llvm.org/bugs/show_bug.cgi?id=27426

Patch by Kirill Bobyrev!

Reviewers: alexfh, LegalizeAdulthood, hokein

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D19406

llvm-svn: 267155