docs: use relative link from CLI to core concepts

[mdjermanovic]

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What is the purpose of this pull request? (put an "X" next to an item)

[x] Documentation update
[ ] Bug fix (template)
[ ] New rule (template)
[ ] Changes an existing rule (template)
[ ] Add autofix to a rule
[ ] Add a CLI option
[ ] Add something to the core
[ ] Other, please explain:

Fixes one recently added link between documents to be relative, as we have multiple docs sites.

What changes did you make? (Give an overview)

Updated the link.

Is there anything you'd like reviewers to focus on?

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[bmish]

Good catch, thanks.

I see some other eslint.org links in the docs. Seems like we need a CI check for this. I don't see any markdownlint rules that would help.