Update reference to scrollSource (w3c#6740)

Co-authored-by: Kevin Ellis <kevers@chromium.org>

Author: kevers-google

scroll-animations-1/Overview.bs

@@ -1103,7 +1103,7 @@ interface CSSScrollTimelineRule : CSSRule {
 					fill: "forwards"
 				});
 			let timeline = new ScrollTimeline({
-				scrollSource: document.documentElement,
+				source: document.documentElement,
 				orientation: "vertical",
 			});
 			let animation = new Animation(effect, timeline);