Update isolates.md - TODOs are reversed

[dkbast]

Fixes confusing (because reversed) Todo comments in the mini exercise.

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Contribution guidelines:

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[ericwindmill]

Thanks for catching!

[dkbast]

Looks like some tests are not going through? This change shouldn't have affected anything though, or could it?

Anything that I can do?

Edit - my bad, I read the contributor guide but still missed it - pipeline should go through now

[atsansone]

/gcbrun

[atsansone]

@dkbast : Thanks for your contribution! I'm running the tests now and staging the site. This should be merged in the next hour.

[dart-github-bot]

Visit the preview URL for this PR (updated for commit eda709f):

https://dart-dev--pr5500-patch-4-vpt9kumq.web.app

[parlough]

Thanks for catching this Damian!

And thanks Eric for the review :)