[v3] Fix negative_binomial_distribution

[Paolo Carlini]

... tweaking the fix like this makes for slightly faster repeated calls 
(spares a division) and also makes clearer that we had a plain typo p / 
(1 - p) for (1 - p) / p, grrr.. Double checked Devroye in the meanwhile.

Committed to mainline, will be in 4.6.1.

Paolo.

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