fix superscript in Am2/kg in rmag.plot_mpms_dc()

Currently Am2/kg is not shown with a proper Am<sup>2</sup>/kg in the `rmag.plot_mpms_dc()` bokeh legend which would be preferable:

<img width="480" alt="Image" src="https://github.com/user-attachments/assets/f44313cb-4039-4a00-ba42-1f121f4bd83b" />