Correct misleading documentation

Update the comment for the `iface` argument for the AsyncSniffer class as it was incorrect.

scapy/sendrecv.py

@@ -1029,7 +1029,7 @@ class AsyncSniffer(object):
                      we have to stop the capture after this packet.
                      --Ex: stop_filter = lambda x: x.haslayer(TCP)
         iface: interface or list of interfaces (default: None for sniffing
-               on all interfaces).
+               on the default interface).
         monitor: use monitor mode. May not be available on all OS
         started_callback: called as soon as the sniffer starts sniffing
                           (default: None).