Fix selector-max-compound-selectors document in website

[ybiquitous]

I noticed the [!WARNING] section is broken the our website (stylelint.io) because starting with an inline code (:not()).

Which issue, if any, is this issue related to?

Follow-up to #7544

Is there anything in the PR that needs further explanation?

No, it's self-explanatory. Just a document fix.

[changeset-bot]

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Latest commit: 0a998cd

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[ybiquitous]

This is blocking the release (#7549). When the CI passes, I'll merge it.