add parameters in method on_snapshot of DocumentReference class

Author: HemangChothani

firestore/google/cloud/firestore_v1/document.py

@@ -512,7 +512,7 @@ def on_snapshot(self, callback):
             db = firestore_v1.Client()
             collection_ref = db.collection(u'users')
 
-            def on_snapshot(document_snapshot):
+            def on_snapshot(document_snapshot, changes, read_time):
                 doc = document_snapshot
                 print(u'{} => {}'.format(doc.id, doc.to_dict()))