Make <poolname> in "ceph osd tier --help" clearer (fix issue 8256).

Make <poolname> in "ceph osd tier --help" clearer.

The ceph osd tier --help info on the left aloways says <poolname>.
It is unclear which one to put <tierpool> on the right.

$ceph osd tier --help
osd tier add <poolname> <poolname> {--   add the tier <tierpool> to base pool
force-nonempty}                          <pool>
osd tier add-cache <poolname>            add a cache <tierpool> of size <size>
<poolname> <int[0-]>                     to existing pool <pool>
...

This patch modifies description on the right to tell which <poolname>:

osd tier add <poolname> <poolname> {--   add the tier <tierpool> (the second
 force-nonempty}                          one) to base pool <pool> (the first
                                           one)
...

Fix: http://tracker.ceph.com/issues/8256

Signed-off-by: Yilong Zhao <accelazh@gmail.com>