Update Elasticsearch adapter tutorial document

For those who integrate Calcite to query Elasticsearch, our tutorial seems a little outdated(last update at 2020.9.28). I find some aspects can be improved:

1. SQL example and its final source json should be updated(about filter and sort, and we support 'nulls sort' after CALCITE-4860 )
2. newest version is 7.15.2(cannot upgrade to higher due to the license changing)
3. term 'Elasticsearch' should keep in consistent