fix(demo): Upgrade UI combo box component (#67)

The older version was throwing errors in the JS console. It still seemed
to function, but the noise made debugging more difficult.

demo/index.html

@@ -28,7 +28,7 @@
     </script>
 
     <!-- Load web components used in the demo. -->
-    <script type="module" src="https://cdn.jsdelivr.net/npm/elix@^9/define/AutoCompleteComboBox.js"></script>
+    <script type="module" src="https://cdn.jsdelivr.net/npm/elix@^15/define/AutoCompleteComboBox.js"></script>
 
     <!-- Load the code for the query part of the demo. -->
     <script src="query.js"></script>