fixed link to CODE_OF_CONDUCT.md within CONTRIBUTING.md

fixed link url to navigate to main brew repo instead of forked repo

CONTRIBUTING.md

 # Contributing to Homebrew
-First time contributing to Homebrew? Read our [Code of Conduct](https://github.com/Homebrew/brew/blob/master/CODEOFCONDUCT.md#code-of-conduct).
+First time contributing to Homebrew? Read our [Code of Conduct](https://github.com/Homebrew/brew/blob/master/CODE_OF_CONDUCT.md#code-of-conduct).
 
 ### Report a bug