Remove extraneous nil

Didn't need it, forgot to remove it.

Author: wvu

modules/auxiliary/dos/dns/bind_tkey.rb

@@ -40,7 +40,7 @@ def initialize(info = {})
 
     register_options([
       Opt::RPORT(53),
-      OptAddress.new('SRC_ADDR', [false, 'Source address to spoof', nil])
+      OptAddress.new('SRC_ADDR', [false, 'Source address to spoof'])
     ])
 
     deregister_options('PCAPFILE', 'FILTER', 'SNAPLEN', 'TIMEOUT')