[JavaFX - Part2] Adjust identification and definition of ScrollPane

[laney0808]

Current:

Problem:

1. The text says “ScrollPane” but the image shows scrollbar
2. The fact that ScrollPane is considered a Control maybe confusing to the students because it looks like a container and is also under “container” in Scene Builder although it extends Control class. Proposed:
3. Change the image for ScrollPane to the entire ScrollPane
4. Briefly explain to the students that “ScrollPane” actually extends the Control class although it seems like a container due to its interactive feature.

[damithc]

@laney0808 Go ahead and submit a PR for this.