[1.2.X] Fixed #14205 - Tiny grammar fix in form validation documentation

  
Thanks to zendak for report and patch.

Backport of [13687] from trunk.


git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.2.X@13689 bcc190cf-cafb-0310-a4f2-bffc1f526a37