Dataset cloud-training-demos.bitcoin_blockchain is not publicly available

The page `courses/data-engineering/demos/nested.md` provides a demonstration of working with nested and repeated data in BigQuery, using a bitcoin dataset.

However, the dataset `cloud-training-demos.bitcoin_blockchain` is not publicly accessible. In this page, we can replace `cloud-training-demos.bitcoin_blockchain` with the publicly accessible `bigquery-public-data.bitcoin_blockchain` dataset.