[5.1.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 5df512e53ab12fd8a0c92421a45aa1b664adb166 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,
             ),
         }