Fixed #21829 -- Added default AppConfigs.

Thanks Russell for the report, Marc for the initial patch, Carl for the
final review, and everyone who contributed to the design discussion.

docs/intro/tutorial01.txt

@@ -435,7 +435,7 @@ look like this:
     :filename: mysite/settings.py
 
     INSTALLED_APPS = (
-        'django.contrib.admin.apps.AdminConfig',
+        'django.contrib.admin',
         'django.contrib.auth',
         'django.contrib.contenttypes',
         'django.contrib.sessions',
@@ -444,13 +444,6 @@ look like this:
         'polls',
     )
 
-.. admonition:: Doesn't match what you see?
-
-    If you're seeing ``'django.contrib.admin'`` instead of
-    ``'django.contrib.admin.apps.AdminConfig'``, you're probably using a
-    version of Django that doesn't match this tutorial version.  You'll want
-    to either switch to the older tutorial or the newer Django version.
-
 Now Django knows to include the ``polls`` app. Let's run another command:
 
 .. code-block:: bash