redundant calling of RegisterGreeterHandlerFromEndpoint in main.go

[FrankGangWang]

Your example is great!

I think you made redundant calling of RegisterGreeterHandlerFromEndpoint in mina.go.

BR,

Frank

   err = gw.RegisterGreeterHandlerFromEndpoint(ctx, mux, *postEndpoint, dialOpts)
if err != nil {
	return nil, err
}