Adjust code with the example's comment (#4466)

Author: glihm

samples/snippets/csharp/VS_Snippets_Wpf/PageNavigationServiceSnippets/CSharp/HomePage.xaml.cs

@@ -18,7 +18,7 @@ public HomePage()
             InitializeComponent();
 
             // Don't allow back navigation if no navigation service
-            if (this.NavigationService != null)
+            if (this.NavigationService == null)
             {
                 this.goBackButton.IsEnabled = false;
             }
@@ -34,4 +34,4 @@ void goBackButton_Click(object sender, RoutedEventArgs e)
         }
     }
     //</SnippetGetPageNavigationServiceCODEBEHIND>
-}
+}