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.
We never talk about the difference between NodeList and Array, and in even our most advanced courses never introduce this concept or require it to complete an exercise. On the other hand, the word "list" does emphasize the need to iterate and might help students to remember to use a `for` loop when accessing individual elements after selecting multiple.