DOC: stats: Several corrections to the betaprime docstring:
* fixed a typo;
* fixed a mistake in the formula for the PDF;
* expressed the PDF in terms of beta(a,b) instead of the gamma function.
MAINT: update author name mapping in tools/authors.py
Merge pull request #2688 from WarrenWeckesser/stats-betaprime-typos

DOC: stats: Several corrections to the betaprime docstring: