Fix docstring of the ZeroInflatedPoisson distribution.

The parameterization uses the variable `mu` but the docstring uses
`mu` in some cases and `theta` in other cases. The fix replaces
all `theta` with `mu` to match the argument name for this distribution.

Author: cscheffler

pymc/distributions/discrete.py

@@ -1465,7 +1465,7 @@ class ZeroInflatedPoisson:
 
         f(x \mid \psi, \mu) = \left\{ \begin{array}{l}
             (1-\psi) + \psi e^{-\mu}, \text{if } x = 0 \\
-            \psi \frac{e^{-\theta}\theta^x}{x!}, \text{if } x=1,2,3,\ldots
+            \psi \frac{e^{-\mu}\mu^x}{x!}, \text{if } x=1,2,3,\ldots
             \end{array} \right.
 
     .. plot::