Update teaching.md : Reference as superscript

[RetriedAgain]

Footnote doesn't work in Gitbook, so I at least set the reference of the research as a superscript.
Here the link to the docs where it is for convenience:
https://devdocs.jabref.org/teaching

• Change in CHANGELOG.md described (if applicable)
• Tests created for changes (if applicable)
• Manually tested changed features in running JabRef (always required)
• Screenshots added in PR description (for UI changes)
• Checked documentation: Is the information available and up to date? If not created an issue at https://github.com/JabRef/user-documentation/issues or, even better, submitted a pull request to the documentation repository.

[Siedlerchr]

Thanks!