use cert2json executable

main.go

@@ -121,7 +121,7 @@ Options:
   -h --help                        Show this screen
   -b --build                       Print version and build information and exit
   -v --version                     Print version information and exit
-  --json-converter=JSONCONVERTER   Path to json converter script [default: /usr/bin/cert2json.py]
+  --json-converter=JSONCONVERTER   Path to json converter script [default: /usr/bin/cert2json]
   --listen-address=LISTENADDRESS   Web server address. Check Go net/http documentation [default: any]
   --listen-port=LISTENPORT         Web server port [default: 8000]
   --verbose                        Log also successful connections