title: property update does not exist on type AngularFirestoreCollection<any>
body: <p>I try to do this but get error:</p>
  
  <pre><code>checkOrUnCheckTitle($key: string , flag: boolean) {
      this.toDoList.update($key, { isChecked: flag})  ;
  </code></pre>
  
  <blockquote>
    <p>property update does not exist on type AngularFirestoreCollection&lt;any&gt;</p>
  </blockquote>
  
  <p>What's going wrong?</p>