zapret-pcap: improve comment

lua/zapret-pcap.lua

@@ -18,7 +18,7 @@ end
 
 -- test case : nfqws2 --qnum 200 --debug --lua-init=@zapret-lib.lua --lua-init=@zapret-pcap.lua --writeable=zdir --in-range=a --lua-desync=pcap:file=test.pcap
 -- arg : file=<filename> - file for storing pcap data. if --writeable is specified and filename is relative - append filename to writeable path
--- arg : keep - do not overwrite file
+-- arg : keep - do not overwrite file, append packets to existing
 function pcap(ctx, desync)
 	if not desync.arg.file or #desync.arg.file==0 then
 		error("pcap requires 'file' parameter")