Tweak wording of CHANGES.txt

Author: Adam Cozzette

CHANGES.txt

@@ -4,7 +4,7 @@
   * Fix missing Windows wheel for Python 3.10 on PyPI
 
   Java
-  * Change implementation of UnknownFieldSet (#9371)
+  * Improve performance characteristics of UnknownFieldSet parsing (#9371)
 
 2021-10-28 version 3.19.1 (C++/Java/Python/PHP/Objective-C/C#/Ruby/JavaScript)