title: Firebase Firestore detach a listener doesn't work
body: <p>I (think I) am following the <a href="https://firebase.google.com/docs/firestore/query-data/listen#detach_a_listener" rel="nofollow noreferrer">instructions on how to detach a listener</a> in <code>Firebase</code> <code>Firestore</code>, but in my code the listener remains active.</p>
  
  <pre><code>// attaching a listener
  this.dbTasks.where('completed', '==', true).onSnapshot(data =&gt; {...code...})
  
  // detaching a listener
  this.dbTasks.where('completed', '==', true).onSnapshot(() =&gt; {})
  </code></pre>
  
  <p>What am I missing?</p>