docs(help): change ziglearn.org to zig.guide

[RaphaelPour]

Regarding to archive.org from 12:34:45 May 19, 2024, https://ziglearn.org has changed to https://zig.guide.

[github-actions]

Hello. Thanks for opening a PR on Exercism 🙂

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[ee7]

Thanks for the PR.

I'd handled this in #393, but I hadn't got round to finishing that yet. I'll merge this in the meantime.