Fix typo in RelationAggregationPaginationServlet error response (#11278)

Author: anoadragon453

changelog.d/11278.misc

@@ -0,0 +1 @@
+Fix a small typo in the error response when a relation type other than 'm.annotation' is passed to `GET /rooms/{room_id}/aggregations/{event_id}`.

synapse/rest/client/relations.py

@@ -298,7 +298,9 @@ async def on_GET(
             raise SynapseError(404, "Unknown parent event.")
 
         if relation_type not in (RelationTypes.ANNOTATION, None):
-            raise SynapseError(400, "Relation type must be 'annotation'")
+            raise SynapseError(
+                400, f"Relation type must be '{RelationTypes.ANNOTATION}'"
+            )
 
         limit = parse_integer(request, "limit", default=5)
         from_token_str = parse_string(request, "from")