Fix ruby gen command

protoc-gen-grpc-ruby doesn't work in my machine and google points to this readme as the only occurance...

Author: bluehallu

README.md

@@ -109,7 +109,7 @@ Make sure that your `$GOPATH/bin` is in your `$PATH`.
      protoc -I/usr/local/include -I. \
        -I$GOPATH/src \
        -I$GOPATH/src/github.com/grpc-ecosystem/grpc-gateway/third_party/googleapis \
-       --plugin=protoc-gen-grpc-ruby=grpc_ruby_plugin \
+       --plugin=protoc-gen-grpc=grpc_ruby_plugin \
        --grpc-ruby_out=. \
        path/to/your/service.proto
      ```