[1.0.X] Fixed #9734 -- Typo fix in the custom fields documentation, pointed out by

col16.

Backport of r9557 from trunk.


git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.0.X@9560 bcc190cf-cafb-0310-a4f2-bffc1f526a37