Shorten description in vfs_linux_xfs_sgid manual

Shorten description in vfs_linux_xfs_sgid manual

this fixes a lexgrog parse error, the NAME subheader description
of the vfs_linux_xfs_sgid(8) manual was too long, this will shorten
the description and allow it to be correctly detected by mandb.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=13562

Signed-off-by: Justin Stephenson <jstephen@redhat.com>
Reviewed-by: Andreas Schneider <asn@samba.org>
Reviewed-by: Jeremy Allison <jra@samba.org>
Autobuild-User(master): Jeremy Allison <jra@samba.org>
Autobuild-Date(master): Thu Aug  9 04:06:17 CEST 2018 on sn-devel-144