Clarify ScrollSpy 'activate' event element [docs][v4][has-pr]

Created by: mikejacobson

In the docs for v4 ScrollSpy, it's not clear that the activate.bs.scrollspy event is now triggered on the scroll element rather than on the nav/tab element like it was in v3.

The event Description doesn't mention which element triggers the event, and the sample code for attaching a listener attaches it to element #myScrollSpy, which is not in any of the example markup so it's difficult to tell which element that is.

I think the event Description should specify which element triggers the event (especially since it's changed from v3) and the sample code for attaching a listener should use a relevant selector.

I'll submit a PR with these changes.