doc: missing osd prefix to ioprio config options

http://tracker.ceph.com/issues/9676 Fixes: #9676

Signed-off-by: Loic Dachary <loic-201408@dachary.org>
Merge pull request #2653 from dachary/wip-9676-ioprio-doc

doc: missing osd prefix to ioprio config options

Reviewed-by: Mehdi Abaakouk <sileht@sileht.net>