docs: fix MCQ archetype to use "multiple-answers"

[SujalTripathi]

Lee pointed out in Slack that the mcq archetype might not be using the correct frontmatter type.

Currently, the archetype uses:

type: mcq
multiple-answers: true

This should be changed to:
type: multiple-answers

(and remove the multiple-answers: true field).

I'd like to work on this and send a PR.

[SujalTripathi]

I checked and couldn't find type: mcq in any archetypes, could you please clarify. @leecalcote