Update README.md

Fix broken link

cpp/README.md

@@ -3,6 +3,6 @@
 First you need to install gRPC on your system. Follow the instructions here:
 [https://github.com/grpc/grpc/blob/master/INSTALL](https://github.com/grpc/grpc/blob/master/INSTALL).
 
-After installing, you can proceed to the [gRPC C++ Hello World tutorial](https://github.com/grpc/grpc-common/cpp/helloworld).
+After installing, you can proceed to the [gRPC C++ Hello World tutorial](helloworld/README.md).
 
 A more detailed tutorial is coming soon.