Update readme on example oscillation parameter settings

[thehrh]

Global best-fit values from various NuFIT versions beyond 2.2 are defined in the config files in https://github.com/icecube/pisa/tree/f739ae4280ebdd5dc36d5b5ed0132f1feec34368/pisa_examples/resources/settings/osc, but the readme doesn't reflect that.

It might also be helpful to point out that only the config files for the 2.X fits subject theta23 to ("shifted") spline priors from https://github.com/icecube/pisa/tree/f739ae4280ebdd5dc36d5b5ed0132f1feec34368/pisa_examples/resources/priors.