Minor updates to magnetostatics notebook

[namurphy]

This PR renames magnetic_statics.ipynb → magnetostatics.ipynb, and then makes some minor updates to the text and plotting.

This PR makes some incremental progress towards #1573, but I'm not intending to fully address it in this PR since I'd like to keep the scope limited to some minor improvements prior to the 0.8.0 release.

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Merging #1588 (5c199ae) into main (05e5ce2) will not change coverage.
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##             main    #1588   +/-   ##
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  Coverage   97.18%   97.18%           
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  Files          82       82           
  Lines        7895     7895           
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  Hits         7673     7673           
  Misses        222      222           

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[namurphy]

@rocco8773 — thanks for the review! I made the change you suggested, so it should be all set.

[rocco8773]

Much better. I can actually read the axes now! 😆

Thanks for making the update!