You can use  `$EDITOR`:

    $ $EDITOR filename.txt

But I think most docs use `nano` because if someone's blindly following along, it's a safe bet to use. If the user has decided they actually prefer one editor over another, they'll know enough to replace it with `vim`, `emacs`, etc themselves.

Of course, the *standard* approach is

    $ ed filename.txt

And if you're sadistic but want an innocent-looking command, you can use

    $ edit filename.txt

which invokes `ex`.