Use `-` instead of lots of commas for emphasize-lines directive ranges

[matthewfeickert]

Ah, this is great as I now have learned that even though the docs say "comma separated" they also later say

Emphasize particular lines (comma-separated numbers which can include ranges), e.g. “3,5,7-9”.

Being able to do ranges with - is nice 👍and I'll cleanup the overuse of commas I have in other exercises later.

Originally posted by @matthewfeickert in #23 (comment)