Improve help for backup_incremental_strategy
Improve help for backup_incremental_strategy

Improve wording of the text.

Change-Id: Id1fe79e74094d6fce4a6db3bb5fa99ab46b42223

cp16net/trove,zhujzhuo/openstack-trove,changsimon/trove,zhujzhuo/openstack-trove,mmasaki/trove,mmasaki/trove,changsimon/trove,zhangg/trove,changsimon/trove,fabian4/trove,fabian4/trove,redhat-openstack/trove,cp16net/trove,redhat-openstack/trove,cp16net/trove,zhujzhuo/openstack-trove,mmasaki/trove,hplustree/trove,fabian4/trove,zhangg/trove,hplustree/trove,openstack/trove,redhat-openstack/trove,openstack/trove