datastore/snippets: replace datastore.Done with iterator.Done
datastore/snippets: replace datastore.Done with iterator.Done

danicat/golang-samples,danicat/golang-samples,GoogleCloudPlatform/golang-samples,danicat/golang-samples,adamyanliev/golang-samples,adamyanliev/golang-samples,GoogleCloudPlatform/golang-samples,adamyanliev/golang-samples,adamyanliev/golang-samples,GoogleCloudPlatform/golang-samples