Data Catalog lookup_entry samples need region tags for docs

In which file did you encounter the issue?

https://github.com/GoogleCloudPlatform/python-docs-samples/blob/master/datacatalog/cloud-client/lookup_entry.py

Did you change the file? If so, how?

Yes. Added "# [START datacatalog_lookup_dataset]" and "# [END datacatalog_lookup_dataset]" to method lookup_bigquery_dataset().