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

Priyansh121096 · web-flow · commit 89d7f37ec887 · 2024-01-10T08:06:05.000-08:00
It was missing the `urllib3.util.Retry` import.
diff --git a/README.rst b/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)
