doc: update default osd_objectstore value
doc: update default osd_objectstore value

since dc3319c3c4e2fb58cb1b5e6c60f165ed28260dc8 this should be reflected
in the doc.

Signed-off-by: Guillaume Abrioux <30a9df5a0293a6108675377fa1673f7aa713f915@redhat.com>

fgal/ceph-ansible,fgal/ceph-ansible,ceph/ceph-ansible,ceph/ceph-ansible