Example code was using the wrong class

Obviously wrong as no code uses that, and the header right above uses the correct class name

Microsoft.Toolkit.Uwp.SampleApp/readme.md

@@ -111,7 +111,7 @@ protected override async void OnNavigatedTo(NavigationEventArgs e)
 {
     base.OnNavigatedTo(e);
 
-    Shell.Current.RegisterNewCommand("Image with placeholder", (sender, args) =>
+    SampleController.Current.RegisterNewCommand("Image with placeholder", (sender, args) =>
     {
         AddImage(false, true);
     });
@@ -199,4 +199,4 @@ Use the DocumentationUrl property to add a link to the raw documentation in *sam
 
 ### CodeUrl
 
-The value of CodeUrl is modified when the app is built in release mode. The branch is automaticaly changed to **master**. This allows you to test the link in debug while pointing to dev.
+The value of CodeUrl is modified when the app is built in release mode. The branch is automaticaly changed to **master**. This allows you to test the link in debug while pointing to dev.