Fix log line that was printing undefined value

changelog.d/6278.misc

@@ -0,0 +1 @@
+Update log line to not use an undefined variable.

synapse/handlers/federation.py

@@ -1250,7 +1250,7 @@ def on_make_join_request(self, origin, room_id, user_id):
                 builder=builder
             )
         except AuthError as e:
-            logger.warn("Failed to create join %r because %s", event, e)
+            logger.warn("Failed to create join %s because %s", room_id, e)
             raise e
 
         event_allowed = yield self.third_party_event_rules.check_event_allowed(