Add PR# to CHANGELOG entry

Author: andrewheard

FirebaseVertexAI/CHANGELOG.md

@@ -24,7 +24,7 @@
   (#13721)
 - [changed] The default request timeout is now 180 seconds instead of the
   platform-default value for a `URLRequest`; this timeout may still be
-  customized in `RequestOptions`.
+  customized in `RequestOptions`. (#13722)
 
 # 11.3.0
 - [added] Added `Decodable` conformance for `FunctionResponse`. (#13606)