[docs] Update docs for NegativeBinomial (#45693)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/45693
**Summary**
This commit updates the docstring for
`torch.distributions.NegativeBinomial` to better match actual behaviour.
In particular, the parameter currently documented as probability of
success is actually probability of failure.
**Test Plan**
1) Ran the code from the issue to make sure this is still an issue (it
is)
2) `make html` and viewed the docs in a browser.
*Before*
<img width="879" alt="Captura de Pantalla 2020-10-01 a la(s) 1 35 28 p m" src="https://user-images.githubusercontent.com/4392003/94864456-db3a5680-03f0-11eb-977e-3bab0fb9c206.png">
*After*
<img width="877" alt="Captura de Pantalla 2020-10-01 a la(s) 2 12 24 p m" src="https://user-images.githubusercontent.com/4392003/94864478-e42b2800-03f0-11eb-965a-51493ca27c80.png">
**Fixes**
This commit closes #42449.
Test Plan: Imported from OSS
Reviewed By: robieta
Differential Revision: D24071048
Pulled By: SplitInfinity
fbshipit-source-id: d345b4de721475dbe26233e368af62eb57a47970
