docs: fix outdated $effect.tracking() demo

[Conduitry]

Fixes #13902. Does not address #13903, which we're now leaning toward fixing by changing the behavior rather than changing the docs.

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[changeset-bot]

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Latest commit: cce3281

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[pkg-pr-new]

pnpm add https://pkg.pr.new/svelte@13906

commit: cce3281