docs: Update preferred NBD device syntax

Mention the preferred URI form, especially since NBD is trying to
standardize that form: https://lists.debian.org/nbd/2019/06/msg00012.html



Signed-off-by: Eric Blake <eblake@redhat.com>
Message-Id: <20190903145634.20237-1-eblake@redhat.com>
Reviewed-by: John Snow <jsnow@redhat.com>