[1.2.X] Fixed #14819 -- Corrected typo in serializer docstring. Thanks to...

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

Backport of r14789 from trunk.

git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.2.X@14792 bcc190cf-cafb-0310-a4f2-bffc1f526a37