fix type

packages/slack/src/utils/verifying-is-slack-request.ts

@@ -6,17 +6,12 @@ import { Request, Response, NextFunction } from 'express';
    * See: https://api.slack.com/authentication/verifying-requests-from-slack
    */
 
-interface verifyingIsSlackRequest{
-  (
-    req: Request & {signingSecret?:string; headers:{'x-slack-signature'?:string, 'x-slack-request-timestamp':number}},
-    res: Response,
-    next: NextFunction,
-  )
+type signingSecretType = {
+  signingSecret?:string; headers:{'x-slack-signature'?:string, 'x-slack-request-timestamp':number}
 }
 
 // eslint-disable-next-line max-len
-export const verifyingIsSlackRequest:verifyingIsSlackRequest = (req, res, next):Record<string, any>| void => {
-
+export const verifyingIsSlackRequest = (req : Request & signingSecretType, res:Response, next:NextFunction):Record<string, any>| void => {
   if (req.signingSecret == null) {
     return res.send('No signing secret.');
   }