doc: replace functions with arrow functions

PR-URL: https://github.com/nodejs/node/pull/6203
Reviewed-By: James M Snell <jasnell@gmail.com>

doc/api/errors.markdown

@@ -410,7 +410,7 @@ allowable type. For example, passing a function to a parameter which expects a
 string would be considered a TypeError.
 
 ```js
-require('url').parse(function() { });
+require('url').parse(() => { });
   // throws TypeError, since it expected a string
 ```
 

doc/api/vm.markdown

@@ -222,7 +222,7 @@ context. The primary use case is to get access to the V8 debug object:
 
 ```js
 const Debug = vm.runInDebugContext('Debug');
-Debug.scripts().forEach(function(script) { console.log(script.name); });
+Debug.scripts().forEach((script) => { console.log(script.name); });
 ```
 
 Note that the debug context and object are intrinsically tied to V8's debugger