Update links (#1926)

CODE-OF-CONDUCT.md

@@ -1,3 +1,3 @@
 ## Community Code of Conduct
 
-gRPC follows the [CNCF Code of Conduct](https://github.com/cncf/foundation/blob/master/code-of-conduct.md).
+gRPC follows the [CNCF Code of Conduct](https://github.com/cncf/foundation/blob/main/code-of-conduct.md).

CONTRIBUTING.md

@@ -1,8 +1,8 @@
 # How to contribute
 
 We definitely welcome patches and contributions to grpc-swift! Please read the gRPC
-organization's [governance rules](https://github.com/grpc/grpc-community/blob/master/governance.md)
-and [contribution guidelines](https://github.com/grpc/grpc-community/blob/master/CONTRIBUTING.md) before proceeding.
+organization's [governance rules](https://github.com/grpc/grpc-community/blob/main/governance.md)
+and [contribution guidelines](https://github.com/grpc/grpc-community/blob/main/CONTRIBUTING.md) before proceeding.
 
 Here are some guidelines and information about how to participate.
 

GOVERNANCE.md

@@ -1 +1 @@
-This repository is governed by the gRPC organization's [governance rules](https://github.com/grpc/grpc-community/blob/master/governance.md).
+This repository is governed by the gRPC organization's [governance rules](https://github.com/grpc/grpc-community/blob/main/governance.md).