Fix ValueError message in AlphaBeta

Correct the error message to indicate that alpha and beta parameters must be
non-negative instead of positive.

src/zennit/rules.py

@@ -233,7 +233,7 @@ class AlphaBeta(BasicHook):
  '''
  def __init__(self, alpha=2., beta=1., stabilizer=1e-6, zero_params=None):
  if alpha < 0 or beta < 0:
- raise ValueError("Both alpha and beta parameters must be positive!")
+ raise ValueError("Both alpha and beta parameters must be non-negative!")
  if (alpha - beta) != 1.:
  raise ValueError("The difference of parameters alpha - beta must equal 1!")
  mod_kwargs = {'zero_params': zero_params}