Fix number of possible options in cluster doc

https://doc.nuxeo.com/nxdoc/nuxeo-cluster-scalability-options/

I was just looking at this page, and I think the math on here is really tangled ... The claim is that two characters from an md5 checksum would provide 36 * 36 combinations, but md5 checksums are in hexadecimal, so that would only be 16 * 16 combinations. The next claim is that by nesting two of those folders we get 36 ^ 4 folder combinations, but that would really be 16 ^ 4. So, since you're in hex, that's really only 16^4 * 10^3 = 65.5 million files total at 1K files/ folder. Which is still a lot of files, but not what you're stating on this page — 1.6 billion).

=> fix incorrect numbers