Fixing broken commands in CONTRIBUTING.md
Fixing broken commands in CONTRIBUTING.md

The git remotes did not point to git repositories, and the org name was
incorrectly listed as ruby-central instead of rubycentral.

rubycentral/cfp-app,ruby-no-kai/cfp-app,ruby-no-kai/cfp-app,rubycentral/cfp-app,rubycentral/cfp-app,ruby-no-kai/cfp-app