[Doc] Inconsistent data format between flink-training-repo and learn-flink doc

The data format specified in [flink-training-repo|https://github.com/apache/flink-training/tree/release-1.16] shows that a TaixRide is either a start or an end one with the eventTime.

However, the Java code in [Data Pipelines & ETL|https://nightlies.apache.org/flink/flink-docs-release-1.16/docs/learn-flink/etl/]  tried to get both "startTime" and "endTime". But in fact, those are not defined in flink-training-repo.

Interval rideInterval = new Interval(ride.startTime, ride.endTime); 

I think such inconsistency would puzzle new comers of Flink.

According to the mail list discussion, it is due to a new [pr|https://github.com/apache/flink-training/pull/36] and the doc forgets to update.