Update scroll-node.md (#484)

* Update scroll-node.md

Regarding the last sentence: “As you can see, the `scrollNode`'s underlying view is a `ASScrollNode`.”:
1. I think that `ASScrollNode` should be replaced with `UIScrollView`.
2. But — not sure how can it be seen from this example...

* Update scroll-node.md

Removed the last sentence, as it did not make sense, and probably left there from previous editions.

docs/_docs/scroll-node.md

@@ -76,6 +76,3 @@ scrollNode.layoutSpecBlock = { node, constrainedSize in
 </pre>
 </div>
 </div>
-
-As you can see, the `scrollNode`'s underlying view is a `ASScrollNode`.
-