etcdctl: sync README with etcdctl role command, add prefix example, fix typo
etcdctl: sync README with etcdctl role command, add prefix example, fix typo

Fixes #7951

whuwxl/etcd,fanminshi/etcd,mickep76/etcd,ajityagaty/etcd,dannysauer/etcd,vimalk78/etcd,mqliang/etcd,xiang90/etcd,li-ang/etcd,glevand/coreos--etcd,rnaveiras/etcd,sinsharat/etcd,etcd-io/etcd,armstrongli/etcd,fanminshi/etcd,xiang90/etcd,YuleiXiao/etcd,heyitsanthony/etcd,mqliang/etcd,bdarnell/etcd,visheshnp/etcd,li-ang/etcd,mkumatag/etcd,heyitsanthony/etcd,sebrandon1/etcd,mqliang/etcd,cockroachdb/etcd,mkumatag/etcd,rnaveiras/etcd,cockroachdb/etcd,mkumatag/etcd,whuwxl/etcd,ajityagaty/etcd,visheshnp/etcd,a-robinson/etcd,gyuho/etcd,ajityagaty/etcd,sinsharat/etcd,coreos/etcd,akauppi/etcd,YuleiXiao/etcd,dannysauer/etcd,rnaveiras/etcd,lorneli/etcd,a-robinson/etcd,bluebreezecf/etcd,coreos/etcd,sebrandon1/etcd,gyuho/etcd,armstrongli/etcd,lorneli/etcd,vimalk78/etcd,etcd-io/etcd,bdarnell/etcd,glevand/coreos--etcd,mickep76/etcd,joshix/etcd,bluebreezecf/etcd,etcd-io/etcd,akauppi/etcd,joshix/etcd