Fix lintian error in manpages.

Fixes a lintian I: message about improper use of minus instead
of a hyphen.

The Debian/Ubuntu maintainers have been keeping this as a diff,
and couldn't be bothered to contribute it upstream.


git-svn-id: http://code.djangoproject.com/svn/django/trunk@17808 bcc190cf-cafb-0310-a4f2-bffc1f526a37