Fix link to CODE_OF_CONDUCT.md

CONTRIBUTING.md

@@ -5,7 +5,7 @@ If you're reading this, you're awesome! Thank you for helping us make this proje
 ## Code of Conduct
 
 MUI has adopted the [Contributor Covenant](https://www.contributor-covenant.org/) as its Code of Conduct, and we expect project participants to adhere to it.
-Please read [the full text](/CODE_OF_CONDUCT.md) so that you can understand what actions will and will not be tolerated.
+Please read [the full text](https://github.com/mui/.github/blob/master/CODE_OF_CONDUCT.md) so that you can understand what actions will and will not be tolerated.
 
 ## A large spectrum of contributions