Small documentation example fixup

`Date.today.midnight` will always return a time in the past; I think what you meant was "the next midnight", which is actually midnight tomorrow if you consider midnight the start of the day (as computer languages do.)
@mogest I'd love to see a section on expiration considerations if you're up to it.

Basically giving people a foundation for the trade offs, involving security, API vs. end-user requests etc. Maybe some general thoughts on finding the right expiration timeframe, too.