common/options: fix the description of osd_max_scrubs

common/options: fix the description of osd_max_scrubs

osd_max_scrubs takes effect even though mclock is used.

```
$ ceph config show-with-defaults osd.0 | grep '^osd_max_scrubs' | sed -E -e 's/( +)/ /g'
osd_max_scrubs 3 default
$ ceph config show-with-defaults osd.0 | grep '^osd.* override ' | sed -E -e 's/( +)/ /g'
osd_delete_sleep 0.000000 override
osd_delete_sleep_hdd 0.000000 override
osd_delete_sleep_hybrid 0.000000 override
osd_delete_sleep_ssd 0.000000 override
osd_pg_delete_cost 15728640 override
osd_recovery_sleep 0.000000 override
osd_recovery_sleep_hdd 0.000000 override
osd_recovery_sleep_hybrid 0.000000 override
osd_recovery_sleep_ssd 0.000000 override
osd_scrub_sleep 0.000000 override
osd_snap_trim_sleep 0.000000 override
osd_snap_trim_sleep_hdd 0.000000 override
osd_snap_trim_sleep_hybrid 0.000000 override
osd_snap_trim_sleep_ssd 0.000000 override
```

Signed-off-by: Satoru Takeuchi <satoru.takeuchi@gmail.com>
(cherry picked from commit 153639ca7f47e58b00377d86ba6baadc45431a48)