Fix typo in release notes

Author: Discookie

clang-tools-extra/docs/ReleaseNotes.rst

@@ -317,7 +317,7 @@ Changes in existing checks
 
 - Improved :doc:`bugprone-unsafe-functions
  <clang-tidy/checks/bugprone/unsafe-functions>` check by omitting the custom 
- replacement string, when the Reason starts with the character `>` in the
+ replacement string, when the reason starts with the character `>` in the
  `CustomFunctions` option.
 
 - Improved :doc:`cppcoreguidelines-init-variables