Fix an ancient bug in the DHCP mixin, lands rapid7#3890

Author: HD Moore

lib/msf/core/exploit/dhcp.rb

@@ -21,7 +21,7 @@ def start_service(hash = {}, context = {})
     @dhcp = Rex::Proto::DHCP::Server.new(hash, context)
     print_status("Starting DHCP server") if datastore['VERBOSE']
     @dhcp.start
-    add_socket(@dhcp.socket)
+    add_socket(@dhcp.sock)
     @dhcp
   end
 
@@ -34,4 +34,3 @@ def stop_service
 end
 
 end
-