doc(docs.mockFunctions): use SAME JS module system (#15486)

Co-authored-by: Christoph Nakazawa <[email protected]>

Author: dancer1325

docs/MockFunctions.md

@@ -22,7 +22,7 @@ export function forEach(items, callback) {
 To test this function, we can use a mock function, and inspect the mock's state to ensure the callback is invoked as expected.
 
 ```js title="forEach.test.js"
-const forEach = require('./forEach');
+import { forEach } from './forEach';
 
 const mockCallback = jest.fn(x => 42 + x);