Fix latex typo in DirichletMultionmial docstrings (#5259)

Author: ricardoV94

Diff for: pymc/distributions/multivariate.py

@@ -629,7 +629,7 @@ class DirichletMultinomial(Discrete):
     .. math::
 
         f(x \mid n, a) = \frac{\Gamma(n + 1)\Gamma(\sum a_k)}
-                              {\Gamma(\n + \sum a_k)}
+                              {\Gamma(n + \sum a_k)}
                          \prod_{k=1}^K
                          \frac{\Gamma(x_k +  a_k)}
                               {\Gamma(x_k + 1)\Gamma(a_k)}