Strip function def from code example (#12101)

Author: Dreamsorcerer

docs/client_middleware_cookbook.rst

@@ -60,6 +60,8 @@ efficient to implement without a middleware:
 
 .. literalinclude:: code/client_middleware_cookbook.py
    :pyobject: token_refresh_preemptively_example
+   :lines: 2-
+   :dedent:
 
 Or combine the above approaches to create a more robust solution.