Update proto generation commands in example doc (#1481)

Author: kkishi

examples/README.md

@@ -51,7 +51,10 @@ OPTIONAL - Rebuilding the generated code
 
 ```
 $ go get -a github.com/golang/protobuf/protoc-gen-go
-$
-$ # from this dir; invoke protoc
-$  protoc -I ./helloworld/helloworld/ ./helloworld/helloworld/helloworld.proto --go_out=plugins=grpc:helloworld
+```
+
+3 Rebuild the generated Go code.
+
+```
+$ go generate google.golang.org/grpc/examples/helloworld/...
 ```