[FIX] website_slides: fix join course widget selector

This commit fixes the selector for the slide_course_join widget. This issue
comes from the removal of the selector 'o_wslides_wrap' recently removed
when cleaning the DOM [1].

Commit linked to task ID 1978530 (fixes linked to internal testing).

[1] See e5c7e43c71

addons/website_slides/static/src/js/slides_course_join.js

@@ -66,7 +66,7 @@ var CourseJoinWidget = Widget.extend({
 });
 
 sAnimations.registry.websiteSlidesCourseJoin = sAnimations.Class.extend({
-    selector: '.o_wslides_wrap',
+    selector: '.o_wslides_js_course_join_link',
 
     /**
      * @override