additional slide in jsweb dom.html

Adding a slide about access methods returning single element vs. nodelist in order to help clarify when array syntax is/is not needed.
This is definitely correct, but I typically gloss over the fact that it's a NodeList and just call it an array. Few people know that it's a NodeList, and I worry it adds unnecessary complexity to put that in their head. I could be convinced either way though. What do you think?