Fix TypeScript errors in guides/routing.mdx code snippets

[ArmandPhilippot]

Description (required)

Follow-up of #14089. TL;DR: Improve UX by preventing the appearance of red squiggles (TS) and visual changes (formatting with Astro extension) when the user copies and pastes the code snippet.

Fixes code snippets in guides/routing.mdx:

• TypeScript complains about page without satisfies GetStaticPaths
• because function getStaticPaths has been updated to const getStaticPaths above, the following code snippet should use the same syntax
• missing closing fence
• TS complains about post.frontmatter.tag, we need post: any
• a few formatting changes

Related issues & labels (optional)

• Suggested label: code snippet update

[astrobot-houston]

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en/guides/routing.mdx	Source changed, localizations will be marked as outdated.

Warnings reference

Icon	Description
🔄️	The source for this localization has been updated since the creation of this pull request, make sure all changes in the source have been applied.

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