fix typo in wiki (IVF)

[saareliad]

wasn't sure if it fits for filing an issue or not...
In
https://github.com/facebookresearch/faiss/wiki/Faiss-indexes#cell-probe-method-with-a-flat-index-as-coarse-quantizer
the n*C should be in the numerator instead of the denominator. (edit: well, after 2nd thought it doesn't really matter...)

should be
n * C * nprobe / nlist = nlist * d
then you get
nlist = sqrt(C * n * nprobe)
nlist ~= C' * sqrt(n)

I was looking for production best practices and rules of thumb such as this one - the wiki certainly answered some of my questions.
Do you share somewhere what configurations of FAISS do you use at production (e.g., Instagram)?
Thanks for the wiki!