Fix button click event for adding stretch child in DockPanelSample

Author: Arlodotexe

components/Primitives/samples/DockPanelSample.xaml

@@ -1,4 +1,4 @@
-<!--  Licensed to the .NET Foundation under one or more agreements. The .NET Foundation licenses this file to you under the MIT license. See the LICENSE file in the project root for more information.  -->
+<!--  Licensed to the .NET Foundation under one or more agreements. The .NET Foundation licenses this file to you under the MIT license. See the LICENSE file in the project root for more information.  -->
 <Page x:Class="PrimitivesExperiment.Samples.DockPanelSample"
       xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
       xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
@@ -27,7 +27,7 @@
                     Content="Add Bottom child" />
             <Button Click="AddRightDock"
                     Content="Add Right child" />
-            <Button Click="AddRightDock"
+            <Button Click="AddStretchDock"
                     Content="Add Stretch child" />
             <Button Click="ClearAllDock"
                     Content="Clear all" />