Proposed update from require.ensure() to import()

Hello,
I just started investigating in this area so let me know if my assumptions are wrong. The [Webpack documentation](https://webpack.js.org/guides/code-splitting-async/) linked here states:

```
webpack supports two similar techniques to achieve this goal: using import() (preferred, ECMAScript proposal) and require.ensure() (legacy, webpack specific).
```

I was able to make `import()` work with a very simple change. I'm taking the assumption that we'd update this documentation to take the current preferred method.

Author: rodet

docs/en/advanced/lazy-loading.md

@@ -9,8 +9,8 @@ All we need to do is define our route components as async components:
 
 ``` js
 const Foo = resolve => {
-  // `require.ensure` is webpack's special syntax for a code-split point.
-  require.ensure(['./Foo.vue'], () => {
+  // The `import()` call sets a code-split point.
+  import('./Foo.vue').then(() => {
     resolve(require('./Foo.vue'))
   })
 }