Update Datastore snippet for read only transaction (#1984)
Update Datastore snippet for read only transaction (#1984)

* `datastore_transactional_single_entity_group_read_only`
GoogleCloudPlatform/python-docs-samples,GoogleCloudPlatform/python-docs-samples,GoogleCloudPlatform/python-docs-samples,GoogleCloudPlatform/python-docs-samples