fixed name of variable

styles/readtheorg/js/readtheorg.js

@@ -6,7 +6,7 @@ function collapse_toc_elements_on_click (nav_li_a){
       states. The active attribute is documented in bootstrap.
       https://getbootstrap.com/docs/4.0/components/navbar/#nav
     */
-    $(nav_li_el).parent().toggleClass("active");
+    $(nav_li_a).parent().toggleClass("active");
 }
 
 $( document ).ready(function() {