docs: renamed "etcd storage" into "etcd registry"
docs: renamed "etcd storage" into "etcd registry"

dongsupark/fleet,improbable-io/fleet,endocode/fleet,coreos/fleet,improbable-io/fleet,dalbani/fleet,dalbani/fleet,endocode/fleet,jonboulle/fleet,improbable-io/fleet,dongsupark/fleet,endocode/fleet,giantswarm/fleet,dongsupark/fleet,dalbani/fleet,coreos/fleet,giantswarm/fleet,jonboulle/fleet,coreos/fleet,giantswarm/fleet,jonboulle/fleet