Update install-kubeadm.md

Telnet is a command that really should not be used, as there is too great a chance it could be misused. NetCat, nc, is a better and newer tool for testing single open ports.

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

@@ -69,10 +69,10 @@ For more details please see the [Network Plugin Requirements](/docs/concepts/ext
 ## Check required ports
 These
 [required ports](/docs/reference/ports-and-protocols/)
-need to be open in order for Kubernetes components to communicate with each other. You can use telnet to check if a port is open. For example:
+need to be open in order for Kubernetes components to communicate with each other. You can use NetCat to check if a port is open. For example:
 
 ```shell
-telnet 127.0.0.1 6443
+nc 127.0.0.1 6443
 ```
 
 The pod network plugin you use (see below) may also require certain ports to be