Add missing curly braces in README.md

[akaiser]

As title, correcting a code snippet.

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[devoncarew]

Thanks for the contribution! Can you sign the CLA (link in the comments above)?

[akaiser]

@devoncarew thx! oh I did and the check is 🟢
https://github.com/dart-lang/args/pull/261/checks?check_run_id=20763577039

[devoncarew]

That check was not able to find CLA info for you. Occasionally the system goes down, and from manually checking it looks like you have signed it. We may need to wait a bit for the system to get back on track.