Fix samba_backup help about retpd

[github at samba.org]

New comment by abartlet on Samba Github repository

https://github.com/samba-team/samba/pull/160#issuecomment-379455902
Comment:
Just a heads-up that a project to replace this with a tested 'samba-tool domain backup' and 'samba-tool domain restore' is on-going (a patch was posted, but metze wanted the restore tool done at the same time so that is in progress), so while we can merge this it likely won't stay around long.

The proposal is that those tools will then replace and remove samba_backup.