Documentation: v2: add -- for force new cluster
Documentation: v2: add -- for force new cluster

this doc is inconsistent between --force-new-cluster and
-force-new-cluster. Use the latter as it is the preferred option in
etcd.

gyuho/etcd,joshix/etcd,mqliang/etcd,mkumatag/etcd,li-ang/etcd,rnaveiras/etcd,li-ang/etcd,coreos/etcd,ajityagaty/etcd,cockroachdb/etcd,mkumatag/etcd,etcd-io/etcd,sinsharat/etcd,ajityagaty/etcd,mkumatag/etcd,cockroachdb/etcd,rnaveiras/etcd,xiang90/etcd,joshix/etcd,rnaveiras/etcd,xiang90/etcd,coreos/etcd,mqliang/etcd,vimalk78/etcd,dannysauer/etcd,ajityagaty/etcd,mqliang/etcd,vimalk78/etcd,gyuho/etcd,sinsharat/etcd,etcd-io/etcd,dannysauer/etcd,etcd-io/etcd