Define the default for the contains_url filter param

Fixes https://github.com/matrix-org/matrix-doc/issues/1553

api/client-server/definitions/room_event_filter.yaml

@@ -31,5 +31,5 @@ allOf:
       type: array
     contains_url:
       type: boolean
-    description: If ``true``, includes only events with a url key in their content. If
-      ``false``, excludes those events.
+      description: If ``true``, includes only events with a ``url`` key in their content. If
+        ``false``, excludes those events. Defaults to ``false``.