docs: replace the future reference link with automation (#53)

manifest.ts

@@ -3,7 +3,7 @@ import { Manifest } from "deno-slack-sdk/mod.ts";
 /**
  * The app manifest contains the app's configuration. This
  * file defines attributes like app name and description.
- * https://api.slack.com/future/manifest
+ * https://api.slack.com/automation/manifest
  */
 export default Manifest({
   name: "deno-blank-template",