Fixed some ReST markup in admin installation docs.

There are six steps in activating the Django admin site:

    1. Add :class:``django.contrib.admin`` to your :setting:``INSTALLED_APPS``

    1. Add ``'django.contrib.admin'`` to your :setting:`INSTALLED_APPS`

       setting.

    2. Admin has two dependencies - :class:``django.contrib.auth`` and

       :class:``django.contrib.contenttypes``. If these applications are not

       in your :setting:``INSTALLED_APPS`` list, add them.

    2. Admin has two dependencies - ``django.contrib.auth`` and

       ``django.contrib.contenttypes``. If these applications are not

       in your :setting:`INSTALLED_APPS` list, add them.

    3. Determine which of your application's models should be editable in the

       admin interface.