Fix `get_state_ids_for_event` return type typo to match what the function actually does (#10050)

It looks like a typo copy/paste from `get_state_for_event` above.

changelog.d/10050.misc

+Fix typo in `get_state_ids_for_event` docstring where the return type was incorrect.

synapse/storage/state.py

@@ -540,7 +540,7 @@ class StateGroupStorage:
             state_filter: The state filter used to fetch state from the database.
 
         Returns:
-            A dict from (type, state_key) -> state_event
+            A dict from (type, state_key) -> state_event_id
         """
         state_map = await self.get_state_ids_for_events(
             [event_id], state_filter or StateFilter.all()