Move reference link to the proper file

CONTRIBUTING.md

@@ -117,7 +117,6 @@ Please follow our [Code of Conduct](CODE_OF_CONDUCT.md).
 
 By contributing your code, you agree to license your contribution under the terms of our project [LICENSE](LICENSE) and to sign the [Salesforce CLA](https://cla.salesforce.com/sign-cla).
 
-[cnb]: https://buildpacks.io
 [discussions]: https://github.com/heroku/buildpacks/discussions
 [install-rust]: https://www.rust-lang.org/tools/install
 [issues]: https://github.com/heroku/buildpacks-ruby/issues

README.md

@@ -86,6 +86,7 @@ The `heroku/ruby` buildpack is a standalone buildpack. This is in contrast to so
 Issues and pull requests are welcome. See our [contributing guidelines](CONTRIBUTING.md) if you would like to help.
 
 
+[cnb]: https://buildpacks.io
 [pack-install]: https://buildpacks.io/docs/for-platform-operators/how-to/integrate-ci/pack/
 [classic-buildpack]: https://github.com/heroku/heroku-buildpack-ruby
 [heroku-buildpacks]: https://github.com/heroku/buildpacks