Reduce ambiguity of cbbackupmgr compatibility table

At the bottom of the cbbackupmgr documentation, there is a table showing the compatibility of cbbackupmgr versions with Couchbase Server versions. I think the headings in the left column are the version of cbbackupmgr, but "cbbackupmgr version" is in bold like the headings in the top row. It would be helpful to have a dividing line or to also have a label for "Couchbase Server version" to make it clearer how the table should be interpreted.