Fixed #16815 -- Rectified the code example for hidden_fields and...

Fixed #16815 -- Rectified the code example for hidden_fields and visible_fields in the forms documentation. Thanks, joonas.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@16823 bcc190cf-cafb-0310-a4f2-bffc1f526a37