[README] Make "Validate Retry" code snippet complete (#701)

It was missing the `urllib3.util.Retry` import.

README.rst

@@ -1232,6 +1232,7 @@ If you are using the ``Retry`` features of ``urllib3`` and want to cover scenari
 
     import responses
     from responses import registries
+    from urllib3.util import Retry
 
 
     @responses.activate(registry=registries.OrderedRegistry)