fix the code block for AppWithCallbackAfterRender (#4571)

* fix the code block for AppWithCallbackAfterRender

fix the code block for AppWithCallbackAfterRender by using createRoot method from 'react-dom/client'

* Update 2022-03-08-react-18-upgrade-guide.md

Co-authored-by: dan <dan.abramov@gmail.com>

content/blog/2022-03-08-react-18-upgrade-guide.md

@@ -60,7 +60,7 @@ We've also removed the callback from render, since it usually does not have the
 ```js
 // Before
 const container = document.getElementById('app');
-ReactDOM.render(<App tab="home" />, container, () => {
+render(<App tab="home" />, container, () => {
   console.log('rendered');
 });
 
@@ -74,7 +74,7 @@ function AppWithCallbackAfterRender() {
 }
 
 const container = document.getElementById('app');
-const root = ReactDOM.createRoot(container);
+const root = createRoot(container);
 root.render(<AppWithCallbackAfterRender />);
 ```