update readme code

Author: BenBagBag

1-Authentication/1-sign-in-react/SPA/src/index.js

@@ -1,5 +1,5 @@
 import React from 'react';
-import ReactDOM from 'react-dom/client';
+import { createRoot } from 'react-dom/client';
 import App from './App';
 import { PublicClientApplication, EventType } from '@azure/msal-browser';
 import { msalConfig } from './authConfig';
@@ -27,7 +27,7 @@ msalInstance.addEventCallback((event) => {
     }
 });
 
-const root = ReactDOM.createRoot(document.getElementById('root'));
+const root = createRoot(document.getElementById('root'));
 root.render(
     <App instance={msalInstance}/>
 );