doc: fix invalid HTML in doc template

Move the script tag for dark mode support inside the body tag.

PR-URL: #36930
Reviewed-By: Antoine du Hamel <duhamelantoine1995@gmail.com>
Reviewed-By: Richard Lau <rlau@redhat.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>

doc/template.html

@@ -66,8 +66,7 @@ <h1>Node.js __VERSION__ Documentation</h1>
       </div>
     </div>
   </div>
-</body>
-<script>
+  <script>
     'use strict';
     {
       const kCustomPreference = 'customDarkTheme';
@@ -102,5 +101,6 @@ <h1>Node.js __VERSION__ Documentation</h1>
         });
       }
     }
-</script>
+  </script>
+</body>
 </html>