use emdashes instead of hyphens

In https://github.com/carpentries/lesson-development-training/pull/217#discussion_r1261220706, @zkamvar pointed out that we can use `---` to specify [an emdash](https://www.merriam-webster.com/words-at-play/em-dash-en-dash-how-to-use) in Markdown throughout the lesson. There are several places where an emdash would be more appropriate than a single `-`, and these should be found and replaced next time anyone is making a proofreading pass through the curriculum.

Review the lesson-development-training curriculum and the linked pull-request discussion first. Search the Markdown lesson text for single hyphens used where em dashes are appropriate, then verify that all identified proofreading cases use the intended `---` notation.

Written by the indexing model from the issue text.