Update --on-port doc

help.txt

@@ -14,8 +14,10 @@ Available options:
         The amount of requests to make before exiting the benchmark. If set, duration is ignored.
   -S/--socketPath
         A path to a Unix Domain Socket or a Windows Named Pipe. A URL is still required in order to send the correct Host header and path.
-  --on-port PATH
-        Start the command listed after -- on the command line. When it starts listening on a port, start sending requests to that port with the given URL path.
+  --on-port
+        Start the command listed after -- on the command line. When it starts listening on a port,
+        start sending requests to that port. A URL is still required in order to send requests to
+        the correct path. The hostname can be omitted, `localhost` will be used by default.
   -m/--method METHOD
         The http method to use. default: 'GET'.
   -t/--timeout NUM