Use document.scrollingElement where appropriate (w3c#33)

Fixes w3c#14

Author: stephenmcgruer

Overview.bs

@@ -379,9 +379,9 @@ not by wall-clock time, but by the progress of scrolling in a [=scroll container
 ::  The scrollable element whose scrolling triggers the activation and drives the
     progress of the timeline.
 
-    If this is not specified, the document element of the [=active document=] for the
-    current [=browsing context=], as determined at the time the {{ScrollTimeline}} is
-    constructed, is used.
+    If this is not specified, the {{scrollingElement}} of the [=active document=]
+    for the current [=browsing context=], as determined at the time the
+    {{ScrollTimeline}} is constructed, is used.
 
     Note that the {{scrollSource}}'s {{ownerDocument}} may change over the lifetime
     of the scroll timeline. This does not affect the scroll timeline; the timeline