Fix bikeshed error

Author: birtles

Overview.bs

@@ -340,7 +340,7 @@ dictionary ScrollTimelineOptions {
   FillMode fill = "none";
 };
 
-[Constructor(ScrollTimelineOptions options)]
+[Constructor(optional ScrollTimelineOptions options)]
 interface ScrollTimeline : AnimationTimeline {
   readonly attribute Element scrollSource;
   readonly attribute ScrollDirection orientation;
@@ -665,4 +665,4 @@ triggering complicated the API, we decided it wasn't worth it if you can't use
 it for transitions, so this feature was removed.
 
 The design space for triggering animations is still open. We welcome input
-on this subject.
+on this subject.