Fixed #28139 -- Added another level of headings in the topics index.

docs/topics/index.txt

@@ -5,7 +5,7 @@ Using Django
 Introductions to all the key parts of Django you'll need to know:
 
 .. toctree::
-   :maxdepth: 1
+   :maxdepth: 2
 
    install
    db/index