Make add_url_rule() signature consistent

This caused a mypy error when I was making another typing improvement,
so I am fixing it before committing my other changes.

src/flask/scaffold.py

@@ -443,7 +443,7 @@ class Scaffold:
         view_func: t.Optional[t.Callable] = None,
         provide_automatic_options: t.Optional[bool] = None,
         **options: t.Any,
-    ) -> t.Callable:
+    ) -> None:
         """Register a rule for routing incoming requests and building
         URLs. The :meth:`route` decorator is a shortcut to call this
         with the ``view_func`` argument. These are equivalent: