Add definition of B in documentation for Beta distribution (#6604)

* Add definition of B in documentation for Beta dist

* Define B in Beta distribution and link Wikipedia.

Author: JoKeyser

pymc/distributions/continuous.py

@@ -1042,6 +1042,10 @@ class Beta(UnitContinuous):
        f(x \mid \alpha, \beta) =
            \frac{x^{\alpha - 1} (1 - x)^{\beta - 1}}{B(\alpha, \beta)}
 
+    where :math:`B` is the Beta function.
+
+    For more information, see https://en.wikipedia.org/wiki/Beta_distribution.
+
     .. plot::
         :context: close-figs