Update [slug].astro

[sgalcheung]

Because SSR pages can’t use getStaticPaths(), they can’t receive props. https://docs.astro.build/en/guides/routing/#modifying-the-slug-example-for-ssr

Fixes #1252

• Tested in different browsers (at least Chromium, Safari, and Firefox).
• Added appropriate tests to avoid future regressions.
• Addressed changes to documentation. Update 010-astro.mdx mdx-docs#379

[vercel]

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