Merge pull request #3266 from pseudocody/master

Fixing link to CODE_OF_CONDUCT.md within CONTRIBUTING.md

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).
 
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