Fix using statement order for StyleCop

Author: michael-hawker

Microsoft.Toolkit.Uwp.UI.Controls.Input/ColorPicker/ColorPickerSlider.cs

@@ -2,9 +2,9 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
+using System;
 using Microsoft.Toolkit.Uwp.Helpers;
 using Microsoft.Toolkit.Uwp.UI.Controls.ColorPickerConverters;
-using System;
 using Windows.Foundation;
 using Windows.UI;
 using Windows.UI.Xaml;