Fix some broken links

[seisman]

Closes https://github.com/seismo-learn/website/issues/171.

[github-actions[bot]]

This comment was written by the Continuous Documentation bot!

• Preview: https://seismo-learn.org/sitepreview/seismo-learn/links/fix-404/index.html
• Commit hash: 08f66c0973a1d28cbb03cce2848ca3b42ff0e70f