Update error message for missing required flags (#580)

Make it so that first letter is not capitalized and rephrase
to remove "have/has".

command.go

@@ -827,7 +827,7 @@ func (c *Command) validateRequiredFlags() error {
 	})
 
 	if len(missingFlagNames) > 0 {
-		return fmt.Errorf(`Required flag(s) "%s" have/has not been set`, strings.Join(missingFlagNames, `", "`))
+		return fmt.Errorf(`required flag(s) "%s" not set`, strings.Join(missingFlagNames, `", "`))
 	}
 	return nil
 }