PubSub: Link to correct TimeoutError in futures docs

[plamut]

Closes #9215.

This PR fixes the futures methods' docs that pointed to an incorrect TimeoutError exception docs. The docs now correctly point to concurrent.futures.TimeoutError, as this is what the code actually raises.

I opted to specify the exact exception class directly, rather than using ~.pubsub_v1.TimeoutError - the latter does not find the actual exception class through the chain of imports within the pubsub package.

How to test

Generate the PubSub docs locally, and verify that publisher's and subscriber's futures classes both point to the correct TimeoutError class (in the docs for the result() and exception() methods).