Update documentation mistake with curl API for mutating iteration fields

[zswaff]

Why:

This example in the curl documentation doesn't match the gh api example and is incorrect. Likely a copy/paste error from the previous example.

Not opening an issue.

What's being changed (if available, include any code snippets, screenshots, or gifs):

Just some incorrect documentation.

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Content directory changes

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Source	Preview	Production	What Changed
issues/planning-and-tracking-with-projects/automating-your-project/using-the-api-to-manage-projects.md	fpt ghec	fpt ghec

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fpt: Free, Pro, Team
ghec: GitHub Enterprise Cloud
ghes: GitHub Enterprise Server
ghae: GitHub AE

[cmwilson21]

@zswaff Thanks so much for opening a PR! I'll get this triaged for review ⚡

[skedwards88]

Thanks for this fix! We'll get this merged down for you.

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