[1.2.X] Fixed #14956 - typo in messages docs. thanks alexarsh for the report.

Backport of r15049 from trunk.

git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.2.X@15050 bcc190cf-cafb-0310-a4f2-bffc1f526a37