[doc] Results.plot callback receive 2 args: (query,answer) (#4764)

Fixes:

```python
>>> a.plot(lambda x:x[1].id)
File /home/isidro/ms/webserver/venv/lib/python3.13/site-packages/scapy/plist.py:288, in _PacketList.plot(self, f, lfilter, plot_xy, **kargs)
    286 # Get the list of packets
    287 if lfilter is None:
--> 288     lst_pkts = [f(*e) for e in self.res]
    289 else:
    290     lst_pkts = [f(*e) for e in self.res if lfilter(*e)]

TypeError: <lambda>() takes 1 positional argument but 2 were given
```

Author: isidroas

doc/scapy/usage.rst

@@ -1121,7 +1121,7 @@ We can easily plot some harvested values using Matplotlib. (Make sure that you h
 For example, we can observe the IP ID patterns to know how many distinct IP stacks are used behind a load balancer::
 
     >>> a, b = sr(IP(dst="www.target.com")/TCP(sport=[RandShort()]*1000))
-    >>> a.plot(lambda x:x[1].id)
+    >>> a.plot(lambda q,r: r.id)
     [<matplotlib.lines.Line2D at 0x2367b80d6a0>]
 
 .. image:: graphics/ipid.png