Fix a parenthesis typo

lib/timeline/Core.js

@@ -291,7 +291,7 @@ class Core {
       }
 
       // make sure your target is a timeline element
-      if (!event.target.className.indexOf("timeline") > -1) return;
+      if (!(event.target.className.indexOf("timeline") > -1)) return;
 
       // make sure only one item is added every time you're over the timeline
       if (itemAddedToTimeline) return;