Fix path to main App in React docs (#522)

docs/react.md

@@ -156,7 +156,7 @@ touch app/javascript/App.css app/javascript/App.js
 ```jsx
 import React from 'react';
 import { createRoot } from 'react-dom/client';
-import HelloMessage from './App';
+import HelloMessage from '../App';
 
 const container = document.getElementById('root');
 const root = createRoot(container);