kubeadm: update the long description about 'kubeadm config' command

Author: SataQiu

cmd/kubeadm/app/cmd/config.go

@@ -59,8 +59,8 @@ func newCmdConfig(out io.Writer) *cobra.Command {
 		Long: fmt.Sprintf(dedent.Dedent(`
 			There is a ConfigMap in the %s namespace called %q that kubeadm uses to store internal configuration about the
 			cluster. kubeadm CLI v1.8.0+ automatically creates this ConfigMap with the config used with 'kubeadm init', but if you
-			initialized your cluster using kubeadm v1.7.x or lower, you must use the 'config upload' command to create this
-			ConfigMap. This is required so that 'kubeadm upgrade' can configure your upgraded cluster correctly.
+			initialized your cluster using kubeadm v1.7.x or lower, you must use the 'kubeadm init phase upload-config' command to 
+			create this ConfigMap. This is required so that 'kubeadm upgrade' can configure your upgraded cluster correctly.
 		`), metav1.NamespaceSystem, constants.KubeadmConfigConfigMap),
 		// Without this callback, if a user runs just the "upload"
 		// command without a subcommand, or with an invalid subcommand,