Fix errors in the suggested index.html template

What was there was an abomination that would not behave at all properly.
The changes I’ve made are extremely conservative. Myself, I’d get rid of
the <html>, <head>, </head>, <body>, </body> and </html> lines as
superfluous, and remove the space inside the div, which I certainly hope
isn’t load-bearing.

docs/reference/src/web/index.md

@@ -38,9 +38,11 @@ $ cargo add dioxus --features web
 Add an `index.html` for Trunk to use. Make sure your "mount point" element has an ID of "main" (this can be configured later):
 
 ```html
+<!DOCTYPE html>
 <html>
   <head>
-    <meta content="text/html;charset=utf-8" http-equiv="Content-Type" name="viewport" content="width=device-width, initial-scale=1.0" charset="UTF-8">
+    <meta charset="utf-8">
+    <meta name="viewport" content="width=device-width, initial-scale=1.0">
   </head>
   <body>
     <div id="main"> </div>