fixed file name

client/src/index.js

@@ -1,7 +1,7 @@
 import React from 'react';
 import ReactDOM from 'react-dom/client';
 import { BrowserRouter } from 'react-router-dom';
-import App from './app';
+import App from './App';
 import reportWebVitals from './reportWebVitals';
 
 const root = ReactDOM.createRoot(document.getElementById('root'));