samples: publish ordering key sample doesn't specify regional endpoint

In the [publishOrderedMessage sample](https://github.com/googleapis/nodejs-pubsub/blob/master/samples/publishOrderedMessage.js), there is currently no region specified. From the [publishing with ordering keys doc](https://cloud.google.com/pubsub/docs/publisher#using_ordering_keys), we need to specify a regional endpoint to publish ordered messages to.