title: How update a reference type in Firestore with Flutter
body: <p>I have a document with 2 fields an integer and a reference types</p>
  
  <p>main 
      value : 10
      ref : products/9ci2hwiMUp63Q9Eugcqg</p>
  
  <p>I want to change the field ref to <code>products/guEgu2sjsyOsSEChu2fT</code>. There is a class <code>DocumentReference</code> and the only attribute to change is a path but is read only attribute.</p>
  
  <p>I read the documentation in firebase.google.com but not exist an example of how to update these type.</p>
  
  <p>Is possible to update this type of field?</p>