DOC: BetaBinomial variance is not correctly described

[aleicazatti]

Issue with current documentation:

In pymc.BetaBinomial docstring the variance is described as follows:

Variance :math:n \dfrac{\alpha \beta}{(\alpha+\beta)^2 (\alpha+\beta+1)}

But, as indicated in the Wikipedia page for the beta-binomial distribution, it should be:

Variance :math:\dfrac{n \alpha \beta (\alpha+\beta+n)}{(\alpha+\beta)^2 (\alpha+\beta+1)}

Idea or request for content:

No response

[aloctavodia]

Thanks for reporting this issue. Do you want to do a PR?