Add a link to the PR in the change log entry

Author: dconeybe

firebase-firestore/CHANGELOG.md

@@ -6,7 +6,7 @@
   the backend.
 - [fixed] Fix a Firestore bug where local cache inconsistencies were
   unnecessarily being resolved, resulting in the incompletion of `Task` objects
-  returned from `get()` invocations.
+  returned from `get()` invocations (#2404).
 
 # (22.0.2)
 - [changed] A write to a document that contains FieldValue transforms is no