Fixed broken link to CODE_OF_CONDUCT.md (#3288)

Author: mcguinlu

CONTRIBUTING.md

@@ -6,7 +6,7 @@ quickly as possible. The guide is divided into two main pieces:
 1. Filing a bug report or feature request in an issue.
 1. Suggesting a change via a pull request.
 
-Please note that ggplot2 is released with a [Contributor Code of Conduct](.github/CODE_OF_CONDUCT.md). By contributing to this project, 
+Please note that ggplot2 is released with a [Contributor Code of Conduct](CODE_OF_CONDUCT.md). By contributing to this project, 
 you agree to abide by its terms.
 
 ## Issues