distinguish operators and functions

feels a bit like nitpicking, but hopefully not too difficult an extra concept to add?

I feel bad about suggesting we introduce a new concept, when we want to keep the learning curve as smooth as possible.... maybe the distinction between functions and operators doesn't really matter... there's always a balance between keeping things simple and glossing over complexities, vs the danger of giving misleading information, and leaving someone confused when they go out to read other guides/tutorials/textbooks.  My gut feeling is that "operator" isn't too hard to understand, and isn't too much to lay onto someone at this stage, but y'all may disagree.

love love love this tutorial btw.