Add missing Box checkbox sample

Author: enisn

sandbox/SandboxMAUI/Pages/CheckBoxPage.xaml

@@ -17,6 +17,7 @@
             <input:CheckBox Text="Option 0 Plain Checkbox" />
             <input:CheckBox Text="Option 1 with Filled Type" Type="Filled" />
             <input:CheckBox Text="Option 2 with Material Type" Type="Material" />
+            <input:CheckBox Text="Option 2 with Square Shape" IconGeometry="{x:Static input:PredefinedShapes.Square}" />
             <input:CheckBox Text="Option 3 with Line Shape" IconGeometry="{x:Static input:PredefinedShapes.Line}" />
             <input:CheckBox Text="Option 3 with Line Shape with Material Type" Type="Material" IconGeometry="{x:Static input:PredefinedShapes.Line}" />
             <input:CheckBox Text="Option 3 with Custom Type (X)" Type="Custom" IconGeometry="M17.705 7.705l-1.41-1.41L12 10.59 7.705 6.295l-1.41 1.41L10.59 12l-4.295 4.295 1.41 1.41L12 13.41l4.295 4.295 1.41-1.41L13.41 12l4.295-4.295z"/>