title: Firestore get DocumentSnapshot's field's value
body: <p>If I have a Firebase Firestore database which I have retrieved a <code>DocumentSnapshot</code> for the document corresponding to the collection on the right and stored in a <code>document</code> variable, then how could I retrieve the value in that <code>DocumentSnapshot</code> at the field "username"?  The field has a string value.</p>
  
  <p><a href="https://i.stack.imgur.com/bQe8s.png" rel="noreferrer"><img src="https://i.stack.imgur.com/bQe8s.png" alt="enter image description here"></a></p>