CONTRIBUTING : fix link to troubleshooting (#133)

The current one returns a 404.

.github/CONTRIBUTING.md

@@ -5,7 +5,7 @@ First time contributing to Homebrew? Read our [Code of Conduct](https://github.c
 
 * run `brew update` (twice)
 * run and read `brew doctor`
-* read [the Troubleshooting Checklist](https://github.com/Homebrew/homebrew/blob/master/share/doc/brew/Troubleshooting.md#troubleshooting)
+* read [the Troubleshooting Checklist](https://github.com/Homebrew/brew/blob/master/share/doc/homebrew/Troubleshooting.md#troubleshooting)
 * open an issue on the formula's repository
 
 Thanks!