Update backup-api.html.md
Update backup-api.html.md

Update backup API feature page to reflect changes in V4 disks API

Signed-off-by: Tal Nisan <1e9a2a77a0973eec628efceb273ea890910487e0@redhat.com>

rafaelmartins/ovirt-site,rafaelmartins/ovirt-site,rafaelmartins/ovirt-site,rafaelmartins/ovirt-site