Edit lesson fix issue due to locale-specific translation of character strings with `strptime`

Lesson specification which uses the parse_text feature in swirldev/swirl#202 to replace a static marker (*i.e.* `-_-`) with text generated using an R expression upon loading the lesson.

Fixes swirldev/swirl#201 but the lesson specification is not backwards compatible (due to the `Parse` field in the question specification).