Fixed #25934 -- Added missing app_name in tutorial 4's urls.py.

docs/intro/tutorial04.txt

@@ -241,6 +241,7 @@ First, open the ``polls/urls.py`` URLconf and change it like so:
 
     from . import views
 
+    app_name = 'polls'
     urlpatterns = [
         url(r'^$', views.IndexView.as_view(), name='index'),
         url(r'^(?P<pk>[0-9]+)/$', views.DetailView.as_view(), name='detail'),