Converted an import statement in an example to Scapy v2

doc/scapy/usage.rst

@@ -931,7 +931,7 @@ This program uses the ``sniff()`` callback (paramter prn). The store parameter i
 ::
 
     #! /usr/bin/env python
-    from scapy import *
+    from scapy.all import *
     
     def arp_monitor_callback(pkt):
         if ARP in pkt and pkt[ARP].op in (1,2): #who-has or is-at