Fix expression for the variance in ZINB docstring

Author: aloctavodia

pymc/distributions/mixture.py

@@ -764,7 +764,9 @@ def ZeroInfNegBinom(a, m, psi, x):
     ========  ==========================
     Support   :math:`x \in \mathbb{N}_0`
     Mean      :math:`\psi\mu`
-    Var       :math:`\psi\mu +  \left (1 + \frac{\mu}{\alpha} + \frac{1-\psi}{\mu} \right)`
+    Var       .. math::
+                  \psi \left(\frac{{\mu^2}}{{\alpha}}\right) +\
+                  \psi \mu + \psi \mu^2 - \psi^2 \mu^2
     ========  ==========================
 
     The zero inflated negative binomial distribution can be parametrized