doc: Fix cache tier docs

The instructions for 'osd tier add' were plain
wrong, claiming that the cache mode could
be passed as a third argument.

Signed-off-by: John Spray <john.spray@inktank.com>
Merge pull request #1877 from ceph/wip-tier-doc

doc: Fix cache tier docs

Reviewed-by: Loic Dachary <loic@dachary.org>