Update outdated URL in code-splitting.md (#2870)

* Update outdated URL

Next.js Dynamic Import.

* Update code-splitting.md

Co-authored-by: Alexey Pyltsyn <lex61rus@gmail.com>

Author: dnhn

content/docs/code-splitting.md

@@ -97,7 +97,7 @@ import("./math").then(math => {
 When Webpack comes across this syntax, it automatically starts code-splitting
 your app. If you're using Create React App, this is already configured for you
 and you can [start using it](https://facebook.github.io/create-react-app/docs/code-splitting) immediately. It's also supported
-out of the box in [Next.js](https://github.com/zeit/next.js/#dynamic-import).
+out of the box in [Next.js](https://nextjs.org/docs/advanced-features/dynamic-import).
 
 If you're setting up Webpack yourself, you'll probably want to read Webpack's
 [guide on code splitting](https://webpack.js.org/guides/code-splitting/). Your Webpack config should look vaguely [like this](https://gist.github.com/gaearon/ca6e803f5c604d37468b0091d9959269).