fix minimum_rotated_rectangle comment

[ljwolf]

This addresses the MRR comment. I think it's better to show how to use this inside of pygeos, rather than make a new release of centrography for this feature. I think that, in general, it may make sense to push towards geopandas/pygeos for these functionalities (e.g. pygeos.minimum_bounding_circle), but we should not rewrite the chapters for this at the moment...