Make documentation consistent with code

[grst]

Update full and reduced scale parameters in documentation to be consistent with code:

diffxpy/diffxpy/testing/tests.py

Lines 862 to 865 in dfe8304

	full_formula_loc = '~ 1 + grouping'
	full_formula_scale = '~ 1'
	reduced_formula_loc = '~ 1'
	reduced_formula_scale = '~ 1'

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