chore: update CHANGELOG.md to add detail and credit on a security-related bugfix in a past version (#1447)

Co-authored-by: Anthonios Partheniou <[email protected]>

Author: andrewsg

CHANGELOG.md

@@ -79,7 +79,7 @@ Please consult the README for details on this major version release.
 
 ### Bug Fixes
 
-* Properly escape URL construction for XML MPU API ([#1333](https://github.com/googleapis/python-storage/issues/1333)) ([bf4d0e0](https://github.com/googleapis/python-storage/commit/bf4d0e0a2ef1d608d679c22b13d8f5d90b39c7b2))
+* Properly escape URL construction for XML MPU API, fixing a path traversal issue that allowed uploads to unintended buckets. Reported by @jdomeracki. ([#1333](https://github.com/googleapis/python-storage/issues/1333)) ([bf4d0e0](https://github.com/googleapis/python-storage/commit/bf4d0e0a2ef1d608d679c22b13d8f5d90b39c7b2))
 
 ## [2.18.0](https://github.com/googleapis/python-storage/compare/v2.17.0...v2.18.0) (2024-07-09)