Fix error message of os-cells sync_instances api

Because parameters of this api are 'updated_since' , 'project_id'
and 'deleted',add 'deleted' to the message "Only 'updated_since'
and 'project_id' are understood."

Change-Id: Ic2b2314861030b029be5df490e123d8976711193
Closes-Bug: #1237836
Merge "Fix error message of os-cells sync_instances api"