Correcting error in kubeadm join syntax

On k8s 1.9, using --discovery-token 
kubeadm join --discovery-token abcdef.1234567890abcdef --discovery-token-unsafe-skip-ca-verification 1.2.3.4:6443`
returns  an error, but the node joins successfully with 
kubeadm join --token abcdef.1234567890abcdef --discovery-token-unsafe-skip-ca-verification 1.2.3.4:6443`

Author: garethcmurphy

Diff for: docs/reference/setup-tools/kubeadm/kubeadm-join.md

@@ -83,7 +83,7 @@ using one of the other modes if possible.
 **Example `kubeadm join` command:**
 
 ```
-kubeadm join --discovery-token abcdef.1234567890abcdef --discovery-token-unsafe-skip-ca-verification 1.2.3.4:6443`
+kubeadm join --token abcdef.1234567890abcdef --discovery-token-unsafe-skip-ca-verification 1.2.3.4:6443`
 ```
 
 **Advantages:**