docs: update_subscription and update_snapshot examples are wrong

The [example](https://github.com/googleapis/python-pubsub/blob/cf9e87c80c0771f3fa6ef784a8d76cb760ad37ef/google/cloud/pubsub_v1/gapic/subscriber_client.py#L528) subscription object doesn't have a 'name' field.  Same with [update_snapshot](https://github.com/googleapis/python-pubsub/blob/cf9e87c80c0771f3fa6ef784a8d76cb760ad37ef/google/cloud/pubsub_v1/gapic/subscriber_client.py#L1496).