docs: add missing 'at' in sentence three of description

[sideshowlol]

Adds a missing 'at' in the third sentence of the description for clarity.

This is a small documentation typo fix and does not affect functionality or code logic.

[changeset-bot]

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Latest commit: d376196

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[svelte-docs-bot]

Preview: https://svelte-dev-git-preview-svelte-16027-svelte.vercel.app/

[Rich-Harris]

thanks!