don't prefix with Printf when already opened

Author: yasunariw

lib/api_remote.ml

@@ -57,7 +57,7 @@ module Slack : Api.Slack = struct
     | Some access_token ->
       let url = "https://slack.com/api/chat.postMessage" in
       let data = Slack_j.string_of_post_message_req msg in
-      let headers = [ Printf.sprintf "Authorization: Bearer %s" access_token ] in
+      let headers = [ sprintf "Authorization: Bearer %s" access_token ] in
       let body = `Raw ("application/json", data) in
       log#info "sending to %s : %s" msg.channel data;
       ( match%lwt http_request ~body ~headers `POST url with