Correct the mongodb_replica_set option's description
Correct the mongodb_replica_set option's description

Current description for mongodb_replica_set is wrong, this patch
fixes it.

Change-Id: I0121beb7a71a282eb73b48d7a7fd4dccb0d36a31
DocImpact

maestro-hybrid-cloud/ceilometer,isyippee/ceilometer,fabian4/ceilometer,sileht/aodh,mathslinux/ceilometer,maestro-hybrid-cloud/ceilometer,eayunstack/ceilometer,isyippee/ceilometer,Juniper/ceilometer,pkilambi/ceilometer,MisterPup/Ceilometer-Juno-Extension,fabian4/ceilometer,r-mibu/ceilometer,chungg/aodh,MisterPup/Ceilometer-Juno-Extension,r-mibu/ceilometer,pczerkas/aodh,eayunstack/ceilometer,Juniper/ceilometer,openstack/ceilometer,mathslinux/ceilometer,openstack/aodh,openstack/aodh,chungg/aodh,redhat-openstack/ceilometer,cernops/ceilometer,sileht/aodh,ityaptin/ceilometer,openstack/ceilometer,pczerkas/aodh,cernops/ceilometer,redhat-openstack/ceilometer,idegtiarov/ceilometer,ityaptin/ceilometer,idegtiarov/ceilometer,pkilambi/ceilometer