docs: fix return type in fetch docstring (#594)

fixes: #576

Co-authored-by: Andrew Gorcester <gorcester@google.com>

Author: cguardia

packages/google-cloud-ndb/google/cloud/ndb/query.py

@@ -1731,7 +1731,7 @@ def fetch(self, limit=None, **kwargs):
                 values for some of these arguments.
 
         Returns:
-            List([model.Model]): The query results.
+            List[Union[model.Model, key.Key]]: The query results.
         """
         return self.fetch_async(_options=kwargs["_options"]).result()