Refactor netx.html to use arrow function for window.onload and add co…

…nst declaration for app element

rust_http_proxy/html/netx.html

@@ -228,7 +228,8 @@
           return value;
         }
 
-        window.onload = function () {
+        window.onload = ()=> {
+          const app = document.getElementById("app")
           const root = ReactDOM.createRoot(app);
           root.render(<App />);
         };