Fixed #7839 -- Corrected admin documentation and tutorial to reflect that...

Fixed #7839 -- Corrected admin documentation and tutorial to reflect that 'classes' should be a list and not a string. Thanks Jan Rademaker for the patch.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@7983 bcc190cf-cafb-0310-a4f2-bffc1f526a37