[5.2.x] Simplified Intersphinx configuration example.

docs.djangoproject.com had been updated to serve the object.inv file
from the default location, so the second tuple element can be None
(the "default" value).

Backport of 5df512e53a from main.

docs/intro/reusable-apps.txt

@@ -276,7 +276,7 @@ this. For a small app like polls, this process isn't too difficult.
             # ...
             "django": (
                 "https://docs.djangoproject.com/en/stable/",
-                "https://docs.djangoproject.com/en/stable/_objects/",
+                None,
             ),
         }