Fix changelog entry for dramatiq_queue_prefetch

[LincolnPuzey]

I believe dramatiq_queue_prefetch is per worker process, not per thread.

As per the source code

dramatiq/dramatiq/worker.py

Line 41 in b372f43

#: The number of messages to prefetch from the queue for each worker.

And this comment #392 (comment)

This matches my testing where, when running with 2 threads, I needed to set dramatiq_queue_prefetch=2 to make each thread process a message, but not to pre-fetch any extra ones.