[scroll-animations] `ScrollTimeline` constructor prose about default `source` should not link to the `scrollingElement` getter but rather a procedure to determine the scrolling element

[graouts]

Currently, the ScrollTimeline constructor indicates that lacking an explicit value provided via the options parameter that the source defaults to the document's scrollingElement. The link here points to https://drafts.csswg.org/cssom-view-1/#dom-document-scrollingelement which is the scrollingElement getter. However, ideally we would not reference a piece of API but rather the procedure used to determine the value returned by that API.