Add alternative example using Python's negative slicing

_episodes/02-data.md

@@ -258,6 +258,9 @@ array([[156, 173, 156, 161],
 > > ~~~
 > > border_size = 500
 > > new_image = raw[border_size:raw.shape[0]-border_size, border_size:raw.shape[1]-border_size]
+> > # We could have also written
+> > # new_image = raw[border_size:-border_size, border_size:-border_size]
+> > # making use of python negative indexing - i.e. from the end of the array - works on lists too
 > > plt.imshow(new_image, cmap='gray')
 > > ~~~
 > > {: .language-python }