Fix label of absolute uncertainty plot
Created by: Zaharid
We basically only use uncertainty PDF plots relative to some central value. Yet, if one plots absolute vales for some reason, the y_label reads sigma/sigma_ref which doesn't make much sense since what is plotted is an absolute standard deviation.
The relevant code is
This was flagged since at least
https://github.com/NNPDF/nnpdf/issues/1353#issuecomment-897231907
Is there any reason why we don't call it sigma(x) that I forget, @enocera ?