Fix example date typo
Fix example date typo

I believe this is what was originally intended. It's the only way I can reconcile the documentation (which talks of unique type/value tuples) with the example data.
benjaminhoffman/cohorts,bslatkin/cohorts,bslatkin/cohorts,benjaminhoffman/cohorts,benjaminhoffman/cohorts,bslatkin/cohorts