clientv3/doc: Fix code example
// // expect dial time-out on ipv4 blackhole
// _, err := clientv3.New(clientv3.Config{
// Endpoints: []string{"http://254.0.0.1:12345"},
-// DialTimeout: 2 * time.Second
+// DialTimeout: 2 * time.Second,
// })
//
// // etcd clientv3 >= v3.2.10, grpc/grpc-go >= v1.7.3
