small mistake in "2_Regression_and_Permutation_Importance.ipynb" and lesson 2 of walkwithfastai

[JaviL]

this would be like a PR for a notebook:
in PermutationImportance --> init

we need to replace "self.y = dls.y_names" by "self.y = learn.dls.y_names", for it to run.