docs: fix code sample for using global directive to preserve unused C…

[speasley]

…SS selector

In my testing, the existing demonstration of using :global() for preserving unused CSS selectors does not work. Using the provided syntax achieves the desired preservation.

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[svelte-docs-bot]

Preview: https://svelte-dev-git-preview-svelte-16219-svelte.vercel.app/

[Conduitry]

The current code is correct. We don't want to style all .post elements in the entire document.

[7nik]

The selector is correct selector for that example.
.post :global { p {...} } results in .post.scoped p - unknown elements are in the child content. While :global(.post) { p {...} } results in .post p.scoped - unknown elements are in the parent content.