Fixing ListBox XAML code example syntax (#1408)

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## Description
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The ListBox example is missing `/` in the closing tags of the options

## Motivation and Context
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<!--- If it fixes an open issue, please link to the issue here. -->
Examples shouldn't have invalid syntax

## How Has This Been Tested?
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to -->
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N/A

## Screenshots (if appropriate):
![Screenshot 2023-11-23
102932](https://github.com/microsoft/WinUI-Gallery/assets/8790386/8bc8d159-7369-4d97-a2de-7b496ada9e21)

## Types of changes
<!--- What types of changes does your code introduce? Put an `x` in all
the boxes that apply: -->
- [x] Bug fix (non-breaking change which fixes an issue)
- [ ] New feature (non-breaking change which adds functionality)
- [ ] Breaking change (fix or feature that would cause existing
functionality to change)

WinUIGallery/ControlPages/ListBoxPage.xaml

@@ -29,10 +29,10 @@
             <local:ControlExample.Xaml>
                 <x:String xml:space="preserve">
 &lt;ListBox SelectionChanged="ColorListBox_SelectionChanged" MinWidth="200"&gt;
-    &lt;x:String&gt;Blue&lt;x:String&gt;
-    &lt;x:String&gt;Green&lt;x:String&gt;
-    &lt;x:String&gt;Red&lt;x:String&gt;
-    &lt;x:String&gt;Yellow&lt;x:String&gt;
+    &lt;x:String&gt;Blue&lt;/x:String&gt;
+    &lt;x:String&gt;Green&lt;/x:String&gt;
+    &lt;x:String&gt;Red&lt;/x:String&gt;
+    &lt;x:String&gt;Yellow&lt;/x:String&gt;
 &lt;/ListBox&gt;
                 </x:String>
             </local:ControlExample.Xaml>