docs(samples): Update Datastore snippet for read only transaction
docs(samples): Update Datastore snippet for read only transaction

* `datastore_transactional_single_entity_group_read_only`

GoogleCloudPlatform/php-docs-samples,GoogleCloudPlatform/php-docs-samples,GoogleCloudPlatform/php-docs-samples,GoogleCloudPlatform/php-docs-samples,GoogleCloudPlatform/php-docs-samples