Unstandardize a few em dashes back to double hyphens again

[ebeshero]

@trishaoconnor efficiently standardized just about every double-hyphen in the Guidelines (wow!), but @sydb discovered that these thorough edits were a bit too thorough in some cases, particularly where applied to CMC examples as noted here:

#2155 (comment)

So I'm opening this ticket to put a few of those back to their original form.