Fixed a typo in the admin documentation.

Note that you may not want autodiscovery of ``admin`` modules when using your

own ``AdminSite`` instance since you will likely be importing all the per-app

``admin`` modules in your ``myproject.admin`` module. This means you need to

put ``'django.contrib.admin.app.SimpleAdminConfig'`` instead of

put ``'django.contrib.admin.apps.SimpleAdminConfig'`` instead of

``'django.contrib.admin'`` in your :setting:`INSTALLED_APPS` setting.