make use a little more helpful

cmd/gcd.go

@@ -44,7 +44,7 @@ func gcd(cmd *cobra.Command, args []string) {
 
 // gcdCmd represents the gcd command
 var gcdCmd = &cobra.Command{
-	Use:   "gcd",
+	Use:   "gcd N N [N ...]",
 	Short: "Compute the greatest common denominator of a set of numbers",
 	Long:  `Compute the greatest common denominator of a set of numbers.`,
 	Args:  cobra.MinimumNArgs(2),