Fixed #34621 -- Made admin site header render in <div> tag.

This was problematic for screen reader users because they use headings
to navigate. Having two <h1> is confusing, and the one in the header
wasn’t particularly helpful since it’s the same on all pages.

docs/intro/tutorial07.txt

@@ -367,7 +367,7 @@ a section of code like:
 .. code-block:: html+django
 
     {% block branding %}
-    <h1 id="site-name"><a href="{% url 'admin:index' %}">Polls Administration</a></h1>
+    <div id="site-name"><a href="{% url 'admin:index' %}">Polls Administration</a><div>
     {% endblock %}
 
 We use this approach to teach you how to override templates. In an actual