Fix to avoid annoying firewall dialog on macOS (#1499)

macOS throws up an annoying firewall dialog with the following
question every time you start the route_guide/server:

Do you want the application “server” to accept incoming network connections?

This simple fix of actually typing out `localhost` seems to fix
this problem.

examples/gotutorial.md

@@ -263,7 +263,7 @@ Once we've implemented all our methods, we also need to start up a gRPC server s
 
 ```go
 flag.Parse()
-lis, err := net.Listen("tcp", fmt.Sprintf(":%d", *port))
+lis, err := net.Listen("tcp", fmt.Sprintf("localhost:%d", *port))
 if err != nil {
         log.Fatalf("failed to listen: %v", err)
 }
@@ -274,7 +274,7 @@ grpcServer.Serve(lis)
 ```
 To build and start a server, we:
 
-1. Specify the port we want to use to listen for client requests using `lis, err := net.Listen("tcp", fmt.Sprintf(":%d", *port))`.
+1. Specify the port we want to use to listen for client requests using `lis, err := net.Listen("tcp", fmt.Sprintf("localhost:%d", *port))`.
 2. Create an instance of the gRPC server using `grpc.NewServer()`.
 3. Register our service implementation with the gRPC server.
 4. Call `Serve()` on the server with our port details to do a blocking wait until the process is killed or `Stop()` is called.

examples/route_guide/server/server.go

@@ -209,7 +209,7 @@ func newServer() *routeGuideServer {
 
 func main() {
 	flag.Parse()
-	lis, err := net.Listen("tcp", fmt.Sprintf(":%d", *port))
+	lis, err := net.Listen("tcp", fmt.Sprintf("localhost:%d", *port))
 	if err != nil {
 		log.Fatalf("failed to listen: %v", err)
 	}