Update effects.ts

fix missing async  (ts compiler wanted it)

Author: hipertracker

packages/overmind-website/examples/guide/getstarted/effects.ts

@@ -9,7 +9,7 @@ export default (ts, view) =>
 import { Post } from './state'
     
 export const jsonPlaceholder = {
-  getPosts(): Promise<Post[]> {
+  async getPosts(): Promise<Post[]> {
     return fetch('https://jsonplaceholder.typicode.com/posts')
       .then(response => response.json())
   }