Fix README: Keep manual implementation examples with field syntax

Co-authored-by: jamesmontemagno <[email protected]>

Author: Copilot

Part 2 - MVVM/README.zh-cn.md

@@ -53,8 +53,8 @@ public void OnPropertyChanged([CallerMemberName] string name = null) =>
 ```csharp
 public class BaseViewModel : INotifyPropertyChanged
 {
-    public partial bool IsBusy { get; set; }
-    public partial string Title { get; set; }
+    bool isBusy;
+    string title;
     //...
 }
 ```

Part 2 - MVVM/README.zh-tw.md

@@ -53,8 +53,8 @@ public void OnPropertyChanged([CallerMemberName] string name = null) =>
 ```csharp
 public class BaseViewModel : INotifyPropertyChanged
 {
-    public partial bool IsBusy { get; set; }
-    public partial string Title { get; set; }
+    bool isBusy;
+    string title;
     //...
 }
 ```