add missing bracket to core-pages.html

The sample script did not run due to a missing bracket.

core-pages.html

@@ -22,7 +22,7 @@
       document.addEventListener('click', function(e) {
         var pages = document.querySelector('core-pages');
         pages.selected = (pages.selected + 1) % pages.children.length;
-      };
+      });
     </script>
 
 @group Polymer Core Elements