Fixed #31330 -- Updated flatpages URLconf example to work with APPEND_SLASH.

Regression in df41b5a05d4e00e80e73afe629072e37873e767a.

docs/ref/contrib/flatpages.txt

@@ -79,7 +79,7 @@ to place the pattern at the end of the other urlpatterns::
 
     # Your other patterns here
     urlpatterns += [
-        path('<path:url>', views.flatpage),
+        path('<path:url>/', views.flatpage),
     ]
 
 .. warning::