Update number of contributors
It was fun to brag about >420 contributors, but it's time to move to another round number: Openhub is listing 539 Monero contributors now. So I propose to raise the number to 500. Changed in all languages, and I already changed it in the Dutch translation that's pending. I hope this is uncontroversial, although technically, you could argue that 512 is a better round number.
This request also corrects a capitalization error in English ('english version'), and a spelling error in all languages except Arabic: change 'Z-Cash' to 'Zcash'.