Update React example to use React.createRef

```jsx
import 'pell/dist/pell.css'
import React, { Component } from 'react';
import { init } from 'pell';

export default class App extends Component {
state = { html: null }
root = React.createRef()

componentDidMount () {
this.editor = init({
element: this.root.current,
onChange: html => this.setState({ html }),
actions: ['bold', 'underline', 'italic'],
})
}

render() {
return (

Start with the React example shown in the issue and compare its ref setup with the requested React.createRef usage. Verify that the example still mounts the pell editor and displays the generated HTML after the update.

Written by the indexing model from the issue text.