Definition of eccentricity in documentation is wrong

[drkeoni]

I noticed recently that the definition of molecular eccentricity as described in the python documentation (i.e. https://www.rdkit.org/docs/source/rdkit.Chem.Descriptors3D.html) shows the denominator as pm3**2 when it should be simply pm3 (per the reference cited there).

I've verified that the code itself (RDKit::Descriptors::eccentricity) has the correct definition so there's no actual bug in the code.

I'll submit a pull request for fixing the python docs.