doc: fix CJS-ESM selector in Safari

Safari needs a vendor prefix on the appearance of the checkbox.

PR-URL: #40135
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Antoine du Hamel <duhamelantoine1995@gmail.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>

doc/api_assets/style.css

@@ -780,6 +780,7 @@ kbd {
 }
 
 .js-flavor-selector {
+  -webkit-appearance: none;
   appearance: none;
   float: right;
   background-image: url(./js-flavor-cjs.svg);