Adjust code with example's text (#4550)

The example is aiming to show a "non available" navigation service. The code should then behave as expected.

Author: glihm

samples/snippets/visualbasic/VS_Snippets_Wpf/PageNavigationServiceSnippets/visualbasic/homepage.xaml.vb

@@ -15,7 +15,7 @@ Namespace VisualBasic
             InitializeComponent()
 
             ' Don't allow back navigation if no navigation service
-            If Me.NavigationService IsNot Nothing Then
+            If Me.NavigationService Is Nothing Then
                 Me.goBackButton.IsEnabled = False
             End If
         End Sub
@@ -28,4 +28,4 @@ Namespace VisualBasic
         End Sub
     End Class
     '</SnippetGetPageNavigationServiceCODEBEHIND>
-End Namespace
+End Namespace