Fix typo in dataclasses documentation

[tmke8]

"affect" -> "effect"

As this is just fixing a typo, I believe it does not need an issue number.

[the-knights-who-say-ni]

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@thomkeh

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[tmke8]

I signed the contributor agreement, and am now waiting for it to be processed. Though I'm also completely fine with someone else opening a PR with the same change. I don't really think this contribution passes the threshold of originality to be copyrightable anyway 😄

[bedevere-bot]

@ericvsmith: Please replace # with GH- in the commit message next time. Thanks!

[ericvsmith]

Thanks for the fix. I agree we can skip the CLA for this.