Fixed #13253 -- Updated the help message for dumpdata to indicate that...

Fixed #13253 -- Updated the help message for dumpdata to indicate that individual models can be serialized. Thanks to arowla for the report.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@12893 bcc190cf-cafb-0310-a4f2-bffc1f526a37