doc: fix typo in openstack radosgw integration

s/type-object store/type object-store/

Signed-off-by: Loic Dachary <loic@dachary.org>
Reviewed-by: Josh Durgin <josh.durgin@inktank.com>
doc: radosgw workaround for OpenStack Horizon bug

Signed-off-by: Loic Dachary <loic@dachary.org>
Reviewed-by: Josh Durgin <josh.durgin@inktank.com>