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.)
I'd suggest against that.  That'd expose you to the condition where it'd be the last millisecond of the day and the sgid would be expired.  Sounds like a crazy edge case, but if your site gets one hit per second, this will happen to you on average once every 2.7 years.  You want to make sure that midnight has been reached before expiring anything.