DOC-3278 Update comment for retry policy (apache#1158)

Author: absurdfarce

cassandra/policies.py

@@ -1027,7 +1027,7 @@ def on_request_error(self, query, consistency, error, retry_num):
         `retry_num` counts how many times the operation has been retried, so
         the first time this method is called, `retry_num` will be 0.
 
-        The default, it triggers a retry on the next host in the query plan
+        By default, it triggers a retry on the next host in the query plan
         with the same consistency level.
         """
         # TODO revisit this for the next major