api-ref: fix succeed code and add field for update-backup
api-ref: fix succeed code and add field for update-backup

https://developer.openstack.org/api-ref/block-storage/v3/index.html?expanded=update-a-backup-detail#update-a-backup

1. metadata is present in the response example of "Update a backup",
but is missing from the response parameters.

2. succeed code of update-backup is 200, not 202

Change-Id: I5c6098b1950b361a38d4a2b3a928344a277a3ed0

mahak/cinder,openstack/cinder,openstack/cinder,mahak/cinder