Confusing kappa description in `hill_transformation`

[wd60622]

The docs describe kappa as the half way point, f(kappa) = 0.5, for any value of s and kappa in both the description and the argument description

I think this should just be for "any value of s."

pymc-marketing/pymc_marketing/mmm/transformers.py

Line 927 in 702d7e5

- :math:`\kappa` is the half-saturation point as :math:`f(\kappa) = 0.5` for any value of :math:`s` and :math:`\kappa`.

Reference:
https://www.pymc-marketing.io/en/stable/api/generated/pymc_marketing.mmm.transformers.hill_function.html#pymc_marketing.mmm.transformers.hill_function