[2.0.x] Fixed #28627 -- Added slug converter to some path() examples in docs.

Backport of 6da140724d from master

docs/ref/class-based-views/generic-display.txt

@@ -59,7 +59,7 @@ many projects they are typically the most commonly used views.
         from article.views import ArticleDetailView
 
         urlpatterns = [
-            path('<slug>/', ArticleDetailView.as_view(), name='article-detail'),
+            path('<slug:slug>/', ArticleDetailView.as_view(), name='article-detail'),
         ]
 
     **Example myapp/article_detail.html**: