update WPF sample

[jonsequitur]

This fixes #3146.

[Awsmolak]

Just tested and still getting Error: Unrecognized command or argument '#!dispatcher'.

For this code block in readme.ipynb:

#!wpf
#!dispatcher
using System.Windows.Media;

App.MainWindow.Background = new SolidColorBrush(Colors.Fuchsia);
App.MainWindow.Background

[jonsequitur]

That's an unrelated issue (#3135) which was fixed last week. You won't see that fix in the stable version of VS Code quite yet. We'll release that fix in VS Code Insiders today.