update changelog for messaging (#5581)

Author: charlotteliang

FirebaseMessaging/CHANGELOG.md

@@ -1,8 +1,8 @@
-# unreleased
-- [changed] Updated NSError with a failure reason to give more details on the error.
+# 2020-05 -- v4.4.1
+- [changed] Updated NSError with a failure reason to give more details on the error. (#5511)
 
 # 2020-04 -- v4.4.0
-- [changed] Changed the location of source under FirebaseMessaging folder to fit the current repository organization.
+- [changed] Changed the location of source under FirebaseMessaging folder to fit the current repository organization. (#5476)
 
 # 2020-04 -- v4.3.1
 - [fixed] Fixed an issue that when a token is deleted, the token refresh notificaiton and delegate is not triggered. (#5338)