[1.2.X] Fixed #14493 -- Corrected use of the wrong attribute in the model...

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

Backport of [14300] from trunk.

git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.2.X@14301 bcc190cf-cafb-0310-a4f2-bffc1f526a37