Make documentation consistent with code

[grst]

Update full and reduced scale parameters in documentation to be consistent with code: https://github.com/theislab/diffxpy/blob/dfe8304db9bba93dfc7efebebecdc62512ca6c16/diffxpy/testing/tests.py#L862-L865

While being mathematically equivalent (I think...) I got confused when trying to understand the code.