fix typo

Author: aleicazatti

pymc/distributions/discrete.py

@@ -1454,7 +1454,7 @@ def ZeroInfNegBinom(a, m, psi, x):
     psi : tensor_like of float
         Expected proportion of NegativeBinomial variates (0 < psi < 1)
     mu : tensor_like of float
-        Poission distribution parameter (mu > 0).
+        Poisson distribution parameter (mu > 0).
     alpha : tensor_like of float
         Gamma distribution parameter (alpha > 0).
     p : tensor_like of float