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Fix docstring

Co-Authored-By: Richard van der Hoff <1389908+richvdh@users.noreply.github.com>

synapse/storage/state.py

@@ -338,7 +338,8 @@ class StateGroupStorage(object):
         the old and the new.
 
         Returns:
-            (prev_group, delta_ids), where both may be None.
+            Deferred[Tuple[Optional[int], Optional[list[dict[tuple[str, str], str]]]]]):
+                (prev_group, delta_ids)
         """
 
         return self.stores.main.get_state_group_delta(state_group)