Fixed #14819 -- Corrected typo in serializer docstring. Thanks to stringify...

Fixed #14819 -- Corrected typo in serializer docstring. Thanks to stringify for the report and patch.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@14789 bcc190cf-cafb-0310-a4f2-bffc1f526a37