[1.7.x] Clarified custom header instructions in tutorial 2.

Then, just edit the file and replace

``{{ site_header|default:_('Django administration') }}`` (including the curly

braces) with your own site's name as you see fit. You should end up with

a line like:

a section of code like:

.. code-block:: html+django

    {% block branding %}

    <h1 id="site-name"><a href="{% url 'admin:index' %}">Polls Administration</a></h1>

    {% endblock %}

We use this approach to teach you how to override templates. In an actual

project, you would probably use