quick-start.md updates of Render in DOM section

[dkoudela]

Hi All,
The following code does not work with the latest version (https://github.com/diegomura/react-pdf-site/blob/master/docs/quick-start.md):
ReactDOM.render(<App />, document.getElementById('root'));

I have found following solution:

// Ensure your page has a root element with id='root', e.g.: <div id="root"></div>
const root = ReactDOM.createRoot(document.getElementById('root') as Element);
root.render(<App />);

I can create a merge request if you allow me to do so.
Otherwise, could you please update the documentation?
Thanks.