net: modify example of Dialer

The previous example was unusual because it used (*Dialer).Dial in a goroutine differennt frrom main one and because it is not necessary to receive Conn from the channel since Conn is produced only once, so modify the example to show just how (*Dialer).DialContext is used.

Updates golang#33743

src/net/example_test.go

@@ -40,35 +40,18 @@ func ExampleListener() {
 }
 
 func ExampleDialer() {
-	d := net.Dialer{
-		Timeout: 3 * time.Minute,
-	}
-	ctx, cancel := context.WithCancel(context.Background())
+	var d net.Dialer
+	ctx, cancel := context.WithDeadline(context.Background(), time.Minute)
 	defer cancel()
-	connCh := make(chan net.Conn)
-	go func() {
-		defer close(connCh)
-		conn, err := d.DialContext(ctx, "tcp", "localhost:12345")
-		if err != nil {
-			log.Fatal(err)
-			return
-		}
 
-		connCh <- conn
-	}()
+	conn, err := d.DialContext(ctx, "tcp", "localhost:12345")
+	if err != nil {
+		log.Fatalf("Failed to dial: %v", err)
+	}
+	defer conn.Close()
 
-	for {
-		select {
-		case <-time.After(time.Second):
-			// You can do cancel() here when something wrong happens.
-		case conn, ok := <-connCh:
-			if !ok {
-				return
-			}
-
-			conn.Write([]byte("hello, go"))
-			conn.Close()
-		}
+	if _, err := conn.Write([]byte("Hello, World!")); err != nil {
+		log.Fatal(err)
 	}
 }