etcdctl: fix typo in README
etcdctl: fix typo in README

some examples use endpoint with port `:2379` but the others with `:12379`

xiang90/etcd,rnaveiras/etcd,sebrandon1/etcd,ajityagaty/etcd,cockroachdb/etcd,coreos/etcd,dannysauer/etcd,gyuho/etcd,etcd-io/etcd,gyuho/etcd,cockroachdb/etcd,dannysauer/etcd,bdarnell/etcd,joshix/etcd,li-ang/etcd,mkumatag/etcd,fanminshi/etcd,xiang90/etcd,mqliang/etcd,sebrandon1/etcd,etcd-io/etcd,ajityagaty/etcd,joshix/etcd,bdarnell/etcd,lorneli/etcd,glevand/coreos--etcd,li-ang/etcd,etcd-io/etcd,mkumatag/etcd,mqliang/etcd,fanminshi/etcd,mqliang/etcd,coreos/etcd,rnaveiras/etcd,glevand/coreos--etcd,rnaveiras/etcd,sinsharat/etcd,vimalk78/etcd,lorneli/etcd,mkumatag/etcd,vimalk78/etcd,ajityagaty/etcd,sinsharat/etcd