Fix docs links for /stations/:id/departures

[fspoettel]

The link in the docs TOC did not work for this specific route due to a typo

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[derhuerst]

Thanks for your contribution! It still only works on GitHub, because the built-in Markdown rendering doesn't render headings as fragments anymore. I will fix this as well.

Edit: In the self-hosted /docs page, links will stay broken until markdown-it/markdown-it#28 is resolved.