Fix typo in planet module description

Change-Id: I8513c85407f863a345eb0b044088c84747ed7fe1
Reviewed-on: https://review.openstack.org/18759
Reviewed-by: Jeremy Stanley <fungi@yuggoth.org>
Reviewed-by: Paul Belanger <paul.belanger@polybeacon.com>
Approved: Clark Boylan <clark.boylan@gmail.com>
Reviewed-by: Clark Boylan <clark.boylan@gmail.com>
Tested-by: Jenkins