eap8 charts doc that reference channels, should be channel manifest

During EAP7-2005 work it was identified that EAP8 Helm charts documentation must be evolved for channels field to mention Channel Manifest instead of Channel.

Helm chart work is outside the scope of this RFE EAP7-2005 (done in https://issues.redhat.com/browse/EAP7-1635) .

For reference the eap8 charts doc that reference channels, should be channel manifest: https://github.com/jbossas/eap-charts/blob/eap8/charts/eap8/README.md (search for build.s2i.channels).