Fix README.md

Hi, thank you for sharing haskell koans on github!

Since I'm newbie of haskell I'm sorry if I'm misunderstanding about how to run haskell programs.
But after running `./bin/setup-koans`, I can't find any files in `bin` directory except `koans` and `setup-koans`.
I guess `bin/koans` is the program to start all of koans programs, and it looks work expectedly, so I opened this pull request.