docs(guides) Update code-splitting.md (#2153)

* Update code-splitting.md

Since webpack 4, when using `dynamic import` to resolve commonJS module, it no longer resolve directly to the value of `module.exports`, but create an artificial namespace object to wrap it instead.

* Update code-splitting.md

Author: tiendo1011

src/content/guides/code-splitting.md

@@ -24,6 +24,7 @@ contributors:
   - kcolton
   - efreitasn
   - EugeneHlushko
+  - Tiendo1011
   - byzyk
 related:
   - title: <link rel=”prefetch/preload”> in webpack
@@ -232,7 +233,7 @@ __src/index.js__
 -
 -   // Lodash, now imported by this script
 -   element.innerHTML = _.join(['Hello', 'webpack'], ' ');
-+   return import(/* webpackChunkName: "lodash" */ 'lodash').then(_ => {
++   return import(/* webpackChunkName: "lodash" */ 'lodash').then(({ default: _ }) => {
 +     var element = document.createElement('div');
 +     var _ = _.default;
 +
@@ -249,6 +250,8 @@ __src/index.js__
 + })
 ```
 
+The reason we need `default` is since webpack 4, when importing a CommonJS module, the import will no longer resolve to the value of `module.exports`, it will instead create an artificial namespace object for the CommonJS module, for more information on the reason behind this, read [webpack 4: import() and CommonJs](https://medium.com/webpack/webpack-4-import-and-commonjs-d619d626b655)
+
 Note the use of `webpackChunkName` in the comment. This will cause our separate bundle to be named `lodash.bundle.js` instead of just `[id].bundle.js`. For more information on `webpackChunkName` and the other available options, see the [`import()` documentation](/api/module-methods#import-). Let's run webpack to see `lodash` separated out to a separate bundle:
 
 ``` bash
@@ -272,7 +275,7 @@ __src/index.js__
 ``` diff
 - function getComponent() {
 + async function getComponent() {
--   return import(/* webpackChunkName: "lodash" */ 'lodash').then(_ => {
+-   return import(/* webpackChunkName: "lodash" */ 'lodash').then({ default: _ } => {
 -     var element = document.createElement('div');
 -
 -     element.innerHTML = _.join(['Hello', 'webpack'], ' ');
@@ -281,7 +284,7 @@ __src/index.js__
 -
 -   }).catch(error => 'An error occurred while loading the component');
 +   var element = document.createElement('div');
-+   const _ = await import(/* webpackChunkName: "lodash" */ 'lodash');
++   const { default: _ } = await import(/* webpackChunkName: "lodash" */ 'lodash');
 +
 +   element.innerHTML = _.join(['Hello', 'webpack'], ' ');
 +