Update master branch refs to main

Signed-off-by: Manuel Mendez <[email protected]>

Author: mmlb

CONTRIBUTING.md

@@ -9,7 +9,7 @@ We're so glad you're here.
 
 ### Code of Conduct
 
-Please read and understand the code of conduct found [here](https://github.com/tinkerbell/.github/blob/master/CODE_OF_CONDUCT.md).
+Please read and understand the code of conduct found [here](https://github.com/tinkerbell/.github/blob/main/CODE_OF_CONDUCT.md).
 
 ### DCO Sign Off