Updates ListBoxItem.OnMouseLeftButtonDown to refer to the left mouse button instead of the right (dotnet#10033)

Quick fix for a typo in the documentation where the `ListBoxItem.OnMouseLeftButtonDown` refers to pressing the right mouse button instead of the left.

Author: jake-pauls

xml/System.Windows.Controls/ListBoxItem.xml

@@ -285,7 +285,7 @@
       </Parameters>
       <Docs>
         <param name="e">The event data.</param>
-        <summary>Called when the user presses the right mouse button over the <see cref="T:System.Windows.Controls.ListBoxItem" />.</summary>
+        <summary>Called when the user presses the left mouse button over the <see cref="T:System.Windows.Controls.ListBoxItem" />.</summary>
         <remarks>To be added.</remarks>
       </Docs>
     </Member>