fix patch number

Author: jordan-smith721

source/whats-new.txt

@@ -34,7 +34,7 @@ Learn what's new in:
 What's New in 5.2.1
 -------------------
 
-The 5.1.2 driver patch release includes the following changes:
+The 5.2.1 driver patch release includes the following changes:
 
 - Fixes an issue that caused applications built with Quarkus to throw a
   ``java.lang.UnsatisfiedLinkError`` when initializing ``libmongocrypt``