Add missing code fence

[louisescher]

Description (required)

This PR fixes a code block which was missing the code fence, resulting in broken code highlights on the docs (see https://docs.astro.build/en/reference/routing-reference/#paginate)

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en/reference/routing-reference.mdx	Source changed, localizations will be marked as outdated.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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[yanthomasdev]

Nice catch, thanks!