modules/rhcos-about.adoc: fix links related to RHCOS upgrades
modules/rhcos-about.adoc: fix links related to RHCOS upgrades

Remove links top projectatomic.io and spruce up the upgrade text a
bit.

vikram-redhat/openshift-docs,vikram-redhat/openshift-docs,vikram-redhat/openshift-docs,vikram-redhat/openshift-docs,vikram-redhat/openshift-docs