Fixed #7838 -- Corrected an invalid class name the 'classes' usage in the...

Fixed #7838 -- Corrected an invalid class name the 'classes' usage in the tutorial. Thanks kratorius for catching this.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@7984 bcc190cf-cafb-0310-a4f2-bffc1f526a37