fix: Fix async main function type (#1672)

This should better reflect the already allowed async function as main

src/types.ts

@@ -57,7 +57,7 @@ export type ApplicationFunctionOptions = {
 export type ApplicationFunction = (
   app: Probot,
   options: ApplicationFunctionOptions
-) => void;
+) => void | Promise<void>;
 
 export type ServerOptions = {
   log?: Logger;