update token ttl description

cmd/kubeadm/app/cmd/token.go

@@ -114,7 +114,7 @@ func NewCmdToken(out io.Writer, errW io.Writer) *cobra.Command {
 		},
 	}
 	createCmd.Flags().DurationVar(&tokenDuration,
-		"ttl", kubeadmconstants.DefaultTokenDuration, "The duration before the token is automatically deleted. 0 means 'never expires'.")
+		"ttl", kubeadmconstants.DefaultTokenDuration, "The duration before the token is automatically deleted (e.g. 1s, 2m, 3h). 0 means 'never expires'.")
 	createCmd.Flags().StringSliceVar(&usages,
 		"usages", kubeadmconstants.DefaultTokenUsages, "The ways in which this token can be used. Valid options: [signing,authentication].")
 	createCmd.Flags().StringVar(&description,