Fix constraint language of subscribe query

cmd/hubserver/storage/postgres.go

@@ -29,7 +29,7 @@ func (s *postgres) Subscribe(topic string, sub Subscriber) error {
 INSERT INTO "subscribers"
 	("topic", "callback", "lease_seconds", "secret", "created")
 VALUES ($1, $2, $3, $4, now())
-ON CONFLICT ON CONSTRAINT subscribers_topic_callback
+ON CONFLICT (topic, callback)
 DO UPDATE SET lease_seconds = excluded.lease_seconds,
               secret = excluded.secret,
               updated = excluded.created