The links you need are
Ruby Core http://ruby-doc.org/core
Ruby STDLIB http://ruby-doc.org/stdlib
This is supposed to be fixed in https://github.com/rails/rails.github.com/commit/0097cafbb98f15618de24eca219d0df1611756dd but I’m guessing the main website doesn’t use the site hosted on Github.
To anybody maintaining the main Rails website please update the links to Core and STDLIB docs of Ruby to the above as those will always point to latest stable. Also the current link to STDLIB is broken, so at the least that needs to be fixed.