Fixed #14493 -- Corrected use of the wrong attribute in the model validation...

Fixed #14493 -- Corrected use of the wrong attribute in the model validation example pseudo-code. Thanks to wogan for the report and patch.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@14300 bcc190cf-cafb-0310-a4f2-bffc1f526a37