Fix module paths for volumev3 volume backup commands

https://review.opendev.org/#/c/612751/ renamed volume.v2.backup
to volume.v2.volume_backup. volume.v3 backup commands refer
volume.v2.backup but they were not updated.
This causes "volume backup xxx" commands
when OS_VOLUME_API_VERSION=3 is specified.

Change-Id: Ib897dd483a7963763016ee3f1e8e1c3cc81d0bb0
Story:o 2006284
Task: 35990

Author: amotoki

setup.cfg

@@ -665,12 +665,12 @@ openstack.volume.v3 =
     volume_show = openstackclient.volume.v2.volume:ShowVolume
     volume_unset = openstackclient.volume.v2.volume:UnsetVolume
 
-    volume_backup_create = openstackclient.volume.v2.backup:CreateVolumeBackup
-    volume_backup_delete = openstackclient.volume.v2.backup:DeleteVolumeBackup
-    volume_backup_list = openstackclient.volume.v2.backup:ListVolumeBackup
-    volume_backup_restore = openstackclient.volume.v2.backup:RestoreVolumeBackup
-    volume_backup_set = openstackclient.volume.v2.backup:SetVolumeBackup
-    volume_backup_show = openstackclient.volume.v2.backup:ShowVolumeBackup
+    volume_backup_create = openstackclient.volume.v2.volume_backup:CreateVolumeBackup
+    volume_backup_delete = openstackclient.volume.v2.volume_backup:DeleteVolumeBackup
+    volume_backup_list = openstackclient.volume.v2.volume_backup:ListVolumeBackup
+    volume_backup_restore = openstackclient.volume.v2.volume_backup:RestoreVolumeBackup
+    volume_backup_set = openstackclient.volume.v2.volume_backup:SetVolumeBackup
+    volume_backup_show = openstackclient.volume.v2.volume_backup:ShowVolumeBackup
 
     volume_host_set = openstackclient.volume.v2.volume_host:SetVolumeHost