Fix the incorrect backup_driver configuration

The "backup_driver" option should be configured to
cinder.backup.drivers.ceph.CephBackupDriver instead of
cinder.backup.drivers.ceph.

Change-Id: I22457023c6ad76b508bcbe05e37517c18f1ffc81
Closes-Bug: #1832878

doc/source/reference/storage/external-ceph-guide.rst

@@ -143,11 +143,14 @@ the following configuration:
    backup_ceph_user=cinder-backup
    backup_ceph_chunk_size = 134217728
    backup_ceph_pool=backups
-   backup_driver = cinder.backup.drivers.ceph
+   backup_driver = cinder.backup.drivers.ceph.CephBackupDriver
    backup_ceph_stripe_unit = 0
    backup_ceph_stripe_count = 0
    restore_discard_excess_bytes = true
 
+For more information about the Cinder backup configuration, see
+:cinder-doc:`Ceph backup driver <configuration/block-storage/backup/ceph-backup-driver.html>`.
+
 Next, copy the ``ceph.conf`` file into ``/etc/kolla/config/cinder/``:
 
 .. code-block:: ini