Note on the Power-2 Limb-darkening Law

Recently there has been a renewed interest in the power-2 limb darkening law for modeling exoplanet transits. This law provides a better match to the intensities generated by spherical stellar atmosphere models than other 2-parameter laws. To help facilitate a wider use of the power-2 law we correct a minor error and, expanding on previous work, suggest a parametrization that can improve the sampling required by some numerical methods such as MCMC.