Fix log line that was printing undefined value (#6278)

* commit 'a2276d4d3':

1 files changed, 1 insertions, 1 deletions

diff --git a/synapse/handlers/federation.py b/synapse/handlers/federation.py

index 17fec1348e..2765dfee5a 100644
--- a/synapse/handlers/federation.py
+++ b/synapse/handlers/federation.py
@@ -1250,7 +1250,7 @@ class FederationHandler(BaseHandler):
                 builder=builder
             )
         except AuthError as e:
-            logger.warn("Failed to create join %r because %s", event, e)
+            logger.warn("Failed to create join to %s because %s", room_id, e)
             raise e
 
         event_allowed = yield self.third_party_event_rules.check_event_allowed(