tutorial: Don't advise against cua-mode

As brudgers said in a HN comment†: "If a tutorial tutorial emphasizing
CUA makes [Emacs] adoption rates higher, the tutorial is more
successful". Not that I'm actively advocating --or even mentioning--
cua-mode, but I don't need to recommend against it for the purposes of
this tutorial.

† https://news.ycombinator.com/item?id=5732786