docs: Correct 'net ads dns unregister --help' description

docs: Correct 'net ads dns unregister --help' description

This change corrects the content of 'net ads dns unregister --help'
command. Updated output would be:
Usage:
net ads dns unregister [hostname]
    Remove all IP Address entires for a given
    hostname from the Active Directory server.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=14005

Signed-off-by: Amit Kumar <amitkuma@redhat.com>
Reviewed-by: Andreas Schneider <asn@samba.org>
Reviewed-by: Ralph Boehme <slow@samba.org>
Autobuild-User(master): Andreas Schneider <asn@cryptomilk.org>
Autobuild-Date(master): Wed Jul  3 15:12:50 UTC 2019 on sn-devel-184