doc: fix JS flavor selection

PR-URL: #37791
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>

doc/api_assets/style.css

@@ -794,7 +794,7 @@ kbd {
 .js-flavor-selector:checked {
   background-image: url(./js-flavor-esm.svg);
 }
-.js-flavor-selector:not(:checked) ~ .esm,
+.js-flavor-selector:not(:checked) ~ .mjs,
 .js-flavor-selector:checked ~ .cjs {
   display: none;
 }