adding UnimplementedGreeterServer to server struct (#1973)

The compiler produces an error without this. The error is:

```
cannot use &server literal (type *server) as type helloworld.GreeterServer in argument to helloworld.RegisterGreeterServer:
	*server does not implement helloworld.GreeterServer (missing helloworld.mustEmbedUnimplementedGreeterServer method)
```

docs/docs/tutorials/adding_annotations.md

@@ -117,7 +117,9 @@ import (
 	helloworldpb "github.com/myuser/myrepo/proto/helloworld"
 )
 
-type server struct{}
+type server struct{
+	helloworldpb.UnimplementedGreeterServer
+}
 
 func NewServer() *server {
 	return &server{}