Fix "CONTRIBUTING.md" link

Author: gpleiss

README.md

@@ -95,7 +95,7 @@ If you use GPyTorch, please cite the following papers:
 
 ## Contributing
 
-See the contributing guidelines [CONTRIBUTING.md](https://github.com/cornellius-gp/gpytorch/blob/main/CONTRIBUTING.md)
+See the contributing guidelines [CONTRIBUTING.md](https://github.com/cornellius-gp/gpytorch/blob/master/CONTRIBUTING.md)
 for information on submitting issues and pull requests.