Remove set accessors from the view model (#911)

The view model can be immutable. This matches the text description (and the intent).

Author: BillWagner

csharp/tutorials/nullable-reference-migration/finished/SimpleFeedReader/ViewModels/NewsStoryViewModel.cs

@@ -9,9 +9,9 @@ public class NewsStoryViewModel
         public NewsStoryViewModel(DateTimeOffset published, string title, string uri) =>
             (Published, Title, Uri) = (published, title, uri);
 
-        public DateTimeOffset Published { get; set; }
-        public string Title { get; set; }
-        public string Uri { get; set; }
+        public DateTimeOffset Published { get; }
+        public string Title { get; }
+        public string Uri { get; }
     }
 #nullable restore
     // </SnippetFinishedViewModel>