doc: use arrow functions in util.md sample code

PR-URL: #17459
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Jon Moss <me@jonathanmoss.me>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>

doc/api/util.md

@@ -129,7 +129,7 @@ such a way that it is marked as deprecated.
 ```js
 const util = require('util');
 
-exports.obsoleteFunction = util.deprecate(function() {
+exports.obsoleteFunction = util.deprecate(() => {
   // Do something here.
 }, 'obsoleteFunction() is deprecated. Use newShinyFunction() instead.');
 ```
@@ -1061,7 +1061,7 @@ util.isObject(null);
 // Returns: false
 util.isObject({});
 // Returns: true
-util.isObject(function() {});
+util.isObject(() => {});
 // Returns: false
 ```
 
@@ -1093,7 +1093,7 @@ util.isPrimitive(undefined);
 // Returns: true
 util.isPrimitive({});
 // Returns: false
-util.isPrimitive(function() {});
+util.isPrimitive(() => {});
 // Returns: false
 util.isPrimitive(/^$/);
 // Returns: false