v3client: fix doc to use e.Server
Was passing embed.Etcd instead of etcdserver.EtcdServer.
// }
//
// // wrap the EtcdServer with v3client
-// cli := v3client.New(e)
+// cli := v3client.New(e.Server)
//
// // use like an ordinary clientv3
// resp, err := cli.Put(context.TODO(), "some-key", "it works!")
