fix: update broken documentation example

[IanGallacher]

Updates documentation example to correctly use changes from PR #641

PR #641 change the example to have import { sveltePreprocess } from 'svelte-preprocess' instead of import preprocess from 'svelte-preprocess', but missed one spot where preprocess needed to change to sveltePreprocess.

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Tests

• Run the tests with npm test or pnpm test

[dummdidumm]

Thank you!