README: fix "release" parameter examples
README: fix "release" parameter examples

"CEPH-2-RHEL-7" is a product version, not a release. Use a real release
like "rhceph-2.1" instead.

red-hat-storage/errata-tool,ktdreyer/errata-tool,ktdreyer/errata-tool,red-hat-storage/errata-tool