handbook/jails: Fix thinjail instructions on UFS

The zfs instructions say to clone a snapshot from the skeleton to the
new thinjail. But the UFS instructions say to _make_ the thinjail
directory first and then copy the skeleton into it. This will cause your
new jail to be inside of
/usr/local/jails/containers/thinjail/13.2-RELEASE-skeleton instead of
just /usr/local/jails/containers/thinjail.

Reviewed by: dbaio
Pull Request: https://github.com/freebsd/freebsd-doc/pull/382