PEP 703: Fix typo

[savannahostrowski]

Pretty self-explanatory, this PR updates a typo.

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📚 Documentation preview 📚: https://pep-previews--3956.org.readthedocs.build/

[savannahostrowski]

@ambv I also noticed a mixture of using hyphens in words (e.g., multi-threaded vs. multithreaded). I opted not to update for consistency in this PR, but I'm curious about your thoughts here. Is there an agreed-upon convention here?

[Rosuav]

Not an error; "proviso" is a word here.

[savannahostrowski]

In that case, it's still a typo :)

Provisio isn't a word - proviso or provision is!

[Rosuav]

True true :) I didn't even see the extra letter in there.

[hugovk]

Thanks!