Fix the inline link in http://railsgirlssummerofcode.org/about/ to the code of conduct

The inline link from http://railsgirlssummerofcode.org/about to the code of conduct is presently not working, as it points to http://railsgirlssummerofcode.org/code-of-conduct , but the code of conduct is at http://railsgirlssummerofcode.org/about/code-of-conduct .

This pull request fixes that minor goof.