doc(docs.mockFunctions): use SAME JS module system

Author: dancer1325

docs/MockFunctions.md

@@ -12,11 +12,12 @@ There are two ways to mock functions: Either by creating a mock function to use
 Let's imagine we're testing an implementation of a function `forEach`, which invokes a callback for each item in a supplied array.
 
 ```js title="forEach.js"
-export function forEach(items, callback) {
+function forEach(items, callback) {
   for (const item of items) {
     callback(item);
   }
 }
+module.exports = forEach;
 ```
 
 To test this function, we can use a mock function, and inspect the mock's state to ensure the callback is invoked as expected.