Fixes the flags for nc in example

The `-p` flag for `nc` should not be used with `-l`.

From the manpage of nc:

```
  -l      Used to specify that nc should listen for an incoming connection rather than initiate a connection to a
          remote host.  It is an error to use this option in conjunction with the -p, -s, or -z options.  Addition‐
          ally, any timeouts specified with the -w option are ignored.
```

`nc` unfortunately does not spit an error in this erroneous case.