Move the fix for #155 to unreleased (#640)

Author: wilhuff

firebase-firestore/CHANGELOG.md

@@ -3,11 +3,11 @@
   that were previously incorrectly disallowed are now allowed. For example,
   after reading a document that doesn't exist, you can now set it multiple
   times successfully in a transaction.
-
-# 20.2.0
 - [fixed] Fixed an issue where query results were temporarily missing documents
   that previously had not matched but had been updated to now match the
   query (#155).
+
+# 20.2.0
 - [feature] Added a `@DocumentId` annotation which can be used on a
   `DocumentReference` or `String` property in a POJO to indicate that the SDK
   should automatically populate it with the document's ID.