pubsub: incorrect region tag in async sample

[hongalex]

python-docs-samples/pubsub/cloud-client/subscriber.py

Lines 198 to 200 in 18f766a

	def receive_messages_with_custom_attributes(project_id, subscription_name):
	"""Receives messages from a pull subscription."""
	# [START pubsub_subscriber_sync_pull_custom_attributes]

This sample uses a region tag that says pubsub_subscriber_sync_pull_custom_attributes, but in reality, the messages are being received asynchronously using subscriber.subscribe. We need to update the region tag and referencing docs.

There's a similar issues with the Ruby samples

[anguillanneuf]

@hongalex Shall we standardize the region tag here as pubsub_subscriber_async_pull_custom_attributes for all languages?

[hongalex]

Sounds good to me. Can you update the tracker? Thanks!

[anguillanneuf]

Sure! Just need to make sure we don't delete these tags until changes are published.