fix: prettier fix rss.xml

[therobrob]

Fixes pre-commit issue in the pipelines:

check xml................................................................Failed
- hook id: check-xml
- exit code: 1

  layouts/rss.xml: Failed to xml parse (1:1 Unexpected characters outside the root element: {)

[github-actions]

Hi there! 👋

Thank you for your contribution to the FIP Guide! 🚀
We appreciate your effort in making rail staff travel information more accessible. 🚄

Checklist before merging:

• Added a description to the Pull Request
• Checked the License of new pictures (non-commercial use without attribution)
• Modified content in English
• Modified content in German
• Modified content in French

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