Added missing return type for get_user_delegation_key

Author: zezha-msft

azure-storage-blob/azure/storage/blob/baseblobservice.py

@@ -549,7 +549,8 @@ def get_user_delegation_key(self, key_start_time, key_expiry_time, timeout=None)
             A DateTime value. Indicates when the key stops being valid.
         :param int timeout:
             The timeout parameter is expressed in seconds.
-        :return:
+        :return: The user delegation key.
+        :rtype: ~azure.storage.blob.models.UserDelegationKey
         """
         _validate_not_none('key_start_time', key_start_time)
         _validate_not_none('key_end_time', key_expiry_time)