Two fixes for release 16.0

 * /observationPeriod should have been included with the stats vocabulary from #2564, based on the datacommons [deployment](https://datacommons.org/browser/observationPeriod).
 * /courseLength was the wrong name, we agreed in #3281 on using /courseSchedule (fixed in https://github.com/schemaorg/schemaorg/commit/f48de032664b47272dba0e3135357d10511fecb5).
