update info on ELO rating

[ThorAvaTahr]

it turns out that the description of implementation of ELO rating is not displayed correctly.
see:
https://boardgamearena.com/forum/viewto ... 446#p59838
https://boardgamearena.com/forum/viewto ... ing#p87282

so suggestions:
- please make the 'c' factor more consistent across games (and preferably based on a probabilistic calculation)
- please publish the correct ELO calculation in the info on the site
- please publish the 'c' factor (hopefully only 1 consistent across games, but otherwise per game)