DOC: power in simple_norm API string got a ref that no one asked for

https://docs.astropy.org/en/latest/api/astropy.visualization.mpl_normalize.simple_norm.html#astropy.visualization.mpl_normalize.simple_norm

I don't think that ref link is supposed to be there?

<img width="514" height="68" alt="Image" src="https://github.com/user-attachments/assets/465bc14d-db41-4fbf-95f1-4721bf518907" />

Needs:

* https://github.com/astropy/sphinx-astropy/pull/103