doc: replace function with arrow function

PR-URL: #17304 Reviewed-By: Joyee Cheung <[email protected]> Reviewed-By: Ron Korving <[email protected]> Reviewed-By: Yuta Hiroto <[email protected]> Reviewed-By: Luigi Pinca <[email protected]> Reviewed-By: Daijiro Wachi <[email protected]> Reviewed-By: Gireesh Punathil <[email protected]> Reviewed-By: Jon Moss <[email protected]> Reviewed-By: James M Snell <[email protected]>

Author: Leko

doc/api/util.md

@@ -419,7 +419,7 @@ but may return a value of any type that will be formatted accordingly by
 constutil=require('util');
 
 constobj={foo:'this will not show up in the inspect() output' };
-obj[util.inspect.custom] =function(depth){
+obj[util.inspect.custom] = (depth)=>{
 return{bar:'baz' };
 };
 
@@ -529,7 +529,7 @@ function doSomething(foo, callback){
 // ...
 }
 
-doSomething[util.promisify.custom] =function(foo){
+doSomething[util.promisify.custom] = (foo)=>{
 returngetPromiseSomehow();
 };
 
@@ -544,8 +544,8 @@ standard format of taking an error-first callback as the last argument.
 For example, with a function that takes in `(foo, onSuccessCallback, onErrorCallback)`:
 
 ```js
-doSomething[util.promisify.custom] =function(foo){
-returnnewPromise(function(resolve, reject){
+doSomething[util.promisify.custom] = (foo)=>{
+returnnewPromise((resolve, reject)=>{
 doSomething(foo, resolve, reject);
  });
 };