Clean up desktop's index.html

Initially I wanted to delete only the duplicated `<html>` opening tag,
but then decided to make it a fully valid HTML document.

Passes https://validator.w3.org/nu/#textarea check.

packages/desktop/src/index.html

@@ -1,22 +1,15 @@
 <!DOCTYPE html>
 <html>
-
-<html>
-
     <head>
-    <meta name="viewport" content="width=device-width, initial-scale=1.0" charset="utf-8" />
+        <title>Dioxus app</title>
+        <meta name="viewport" content="width=device-width, initial-scale=1.0" />
     </head>
-
-
     <body>
-    <div id="main">
+        <div id="main"></div>
-    </div>
-</body>
-
         <script>
             import("./index.js").then(function (module) {
                 module.main();
             });
         </script>
-
+    </body>
 </html>