doc: fix branch management link
doc: fix branch management link

Fixes #12875

etcd-io/etcd,rnaveiras/etcd,mkumatag/etcd,ajityagaty/etcd,ajityagaty/etcd,mkumatag/etcd,etcd-io/etcd,gyuho/etcd,ajityagaty/etcd,mkumatag/etcd,mqliang/etcd,coreos/etcd,rnaveiras/etcd,rnaveiras/etcd,mqliang/etcd,mqliang/etcd,coreos/etcd,gyuho/etcd,etcd-io/etcd