refactor: arrow function

Author: curbengh

lib/punycode.js

@@ -227,7 +227,7 @@ const decode = (input) => {
  * string.
  */
 const toUnicode = (input) => {
-  return mapDomain(input, function(string) {
+  return mapDomain(input, (string) => {
     return regexPunycode.test(string)
       ? decode(string.slice(4).toLowerCase())
       : string;