message: docs: fix contributor code of conduct link
diff: [fork]: https://github.com/graphcool/graphql-playground/fork
  [pr]: https://github.com/graphcool/graphql-playground/compare
  -[code-of-conduct]: CODE_OF_CONDUCT.md
  +[code-of-conduct]: ../CODE_OF_CONDUCT.md
  Hi there! We're thrilled that you'd like to contribute to this project. Your help is essential for keeping it great.
  