Update the check for open port in install-kubeadm.md

Tell `nc` to report open ports instead of initiating a connection by
using the `-z` flag, and report connection failure after a timeout of 2
seconds.

content/en/docs/setup/production-environment/tools/kubeadm/install-kubeadm.md

@@ -60,7 +60,7 @@ need to be open in order for Kubernetes components to communicate with each othe
 You can use tools like [netcat](https://netcat.sourceforge.net) to check if a port is open. For example:
 
 ```shell
-nc 127.0.0.1 6443 -v
+nc 127.0.0.1 6443 -zv -w 2
 ```
 
 The pod network plugin you use may also require certain ports to be