[clang-tools-extra] Fix small typo in docs/ReleaseNotes.rst

Summary:
This is mainly a test diff to check the new Herald rule I
added in LLVM Phabricator to automatically Cc: cfe-commits on all
clang-tools-extra diffs.

Reviewers: Wizard, hokein, klimek

Reviewed By: Wizard

Subscribers: dlj, bkramer, sammccall

Differential Revision: https://reviews.llvm.org/D40180

llvm-svn: 319040