Docs fix-up: Remove references to ProcessPoolExecutor from asyncio-dev.rst

[eirrgang]

ddd5f36 updated asyncio-eventloop.rst, but overlooked other references to using non-ThreadPoolExecutors with run_in_executor().

Ref bpo-43234: Prohibit non-ThreadPoolExecutor in loop.set_default_executor (GH-24540)

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📚 Documentation preview 📚: https://cpython-previews--105543.org.readthedocs.build/

[eirrgang]

I think I may have misinterpreted the update and the documentation. It looks like it is only set_default_executor for which ProcessPoolExecutor has been disallowed. A ProcessPoolExecutor instance can still be passed to run_in_executor() as an argument.