[3.0.x] Fixed #31330 -- Corrected catchall URL pattern in flatpages docs.

Use re_path() pattern with the regex used before original regression in
df41b5a05d4e00e80e73afe629072e37873e767a.
Regression in a0916d7212aaae634f4388d47d8717abc2cd9036.

Backport of 8f2a6c76d19e4010c4683b20ed7f1eb4b07c17eb from master

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),
+        re_path(r'^(?P<url>.*/)$', views.flatpage),
     ]
 
 .. warning::