Fix api-ref for POST backup for optional name

name attribute in create backup request is optional but
api-ref does not say so.
- https://developer.openstack.org/api-ref/block-storage/v3/index.html?expanded=create-a-backup-detail#backups-backups

Change-Id: I9ac456382ac8d3a27256762e3a56bd784abda201
Closes-Bug: #1678005

api-ref/source/v3/ext-backups.inc

@@ -228,7 +228,7 @@ Request
    - volume_id: volume_id
    - force: force
    - backup: backup
-   - name: name
+   - name: name_optional
    - snapshot_id: snapshot_id_2
    - project_id: project_id_path
 

api-ref/source/v3/parameters.yaml

@@ -1334,6 +1334,12 @@ name_9:
   in: body
   required: true
   type: string
+name_optional:
+  description: |
+    The name of the Volume Backup.
+  in: body
+  required: false
+  type: string
 namespace:
   description: |
     Link associated to the extension.