Correct RuboCop name references in the README

Btw, the comparison could be extended a bit as RuboCop does much more complex analysis than variable name and method length checks. :-) We've reimplemented all built-in Ruby checks (ruby -w) and improved them in the process. We've also added quite a few lint checks of our own. I'd say the bigger difference is that we're doing static analysis on a per-file level and I guess ruby-lint is doing analysis on a project level (as I cannot imagine another way to check for unused instance variables).