docs: Fixed dead link to MainsailOS

[ThatColdToast]

[JamesH1978]

Looks fine to me, but please also be aware that you need to sign off a PR, please see point 3 in https://github.com/Klipper3d/klipper/blob/master/docs/CONTRIBUTING.md#what-to-expect-in-a-review

Thanks James

[github-actions[bot]]

Thank you for your contribution to Klipper. Unfortunately, a reviewer has not assigned themselves to this GitHub Pull Request. All Pull Requests are reviewed before merging, and a reviewer will need to volunteer. Further information is available at: https://www.klipper3d.org/CONTRIBUTING.html

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[JamesH1978]

@ThatColdToast if you could just add your signed off by line, we can get this approved and merged, if you are unwilling to do this, then i can go ahead a push a PR myself.

Thanks James

[JamesH1978]

Thanks for adding that typo change, but i still dont see a signed off by line, so i can review and accept it.

Thanks James