Fix default for `max-positional-args` (#9838)
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## Summary
`max-positional-args` defaults to `max-args` if it's not specified and
the default to `max-args` is 5, so saying that the default is 3 is
definitely wrong. Ideally, we wouldn't specify a default at all for this
config option, but I don't think that's possible?
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## Test Plan
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Not sure.
