Update doc on max_workers

[xianbaoqian]

Describe the bug

max_workers seems to be deprecated by max_threads

https://github.com/gradio-app/gradio/blob/487db7b5d57e1a1350494efdf37c121cbf627780/gradio/blocks.py#L2037

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Reproduction

import gradio as gr

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Severity

I can work around it