Example should be updated

[scottshotgg]

The example should be updated to reflect changes in the lib. The WithContext function needs to be used now and the returned warning do not need to be de-ref'd.

ctx, cancel := context.WithTimeout(context.Background(), 5*time.Minute)
    defer cancel()

    // Equivalent to `/usr/local/bin/nmap -p 80,443,843 google.com facebook.com youtube.com`,
    // with a 5-minute timeout.
    scanner, err := nmap.NewScanner(
        nmap.WithContext(ctx),
        nmap.WithTargets("google.com", "facebook.com", "youtube.com"),
        nmap.WithPorts("80,443,843"),
    )
    if err != nil {
        log.Fatalf("unable to create nmap scanner: %v", err)
    }

    result, warnings, err := scanner.Run()
    if len(warnings) > 0 {
        log.Printf("run finished with warnings: %s\n", warnings) // Warnings are non-critical errors from nmap.
    }
    if err != nil {
        log.Fatalf("unable to run nmap scan: %v", err)
    }

    // Use the results to print an example output
    for _, host := range result.Hosts {
        if len(host.Ports) == 0 || len(host.Addresses) == 0 {
            continue
        }

        fmt.Printf("Host %q:\n", host.Addresses[0])

        for _, port := range host.Ports {
            fmt.Printf("\tPort %d/%s %s %s\n", port.ID, port.Protocol, port.State, port.Service.Name)
        }
    }

    fmt.Printf("Nmap done: %d hosts up scanned in %.2f seconds\n", len(result.Hosts), result.Stats.Finished.Elapsed)