CONTRIBUTING.md: Fix out-of-date CLA documentation
CONTRIBUTING.md: Fix out-of-date CLA documentation

The CLA system swapped a while back and this was left behind.

ejona86/grpc-java,ejona86/grpc-java,grpc/grpc-java,ejona86/grpc-java,dapengzhang0/grpc-java,stanley-cheung/grpc-java,dapengzhang0/grpc-java,ejona86/grpc-java,grpc/grpc-java,dapengzhang0/grpc-java,grpc/grpc-java,grpc/grpc-java,stanley-cheung/grpc-java,dapengzhang0/grpc-java,stanley-cheung/grpc-java,stanley-cheung/grpc-java