change check_slack arg position

Author: yasunariw

src/monorobot.ml

@@ -91,7 +91,7 @@ let gh_payload =
 
 let slack_payload =
   let doc = "path to a JSON file containing a slack notification payload" in
-  Arg.(required & pos 1 (some file) None & info [] ~docv:"SLACK_PAYLOAD" ~doc)
+  Arg.(required & pos 0 (some file) None & info [] ~docv:"SLACK_PAYLOAD" ~doc)
 
 let json =
   let doc = "if set, will format output as json" in