Fix misleading typo in image caption

Why is this change being made?

I believe the caption for the 2021 is not accurate and should say one queue per worker rather than one queue per shard, which is actually what the next image in the blog post is depicting if my understanding is correct.

I'm reading through all the different sources/issues/mrs of the incredible work GitLab has done in changing the sidekiq architecture and this particular caption was pretty much confusing me (as my understanding is the 'shards' on the image are urgent cpu-bound, catchall etc. and they actually have multiple queues assigned), until the point when I've checked what the referenced .png file is named to confirm (I hope) my suspicion.

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