chore: fix grid composition example (#519) [ci skip]

Commit 27a72bd8f150be956a7931b64590cf3867bb4803 fixed the example but introduced a useless import.

packages/vue/examples/composition/grid.html

@@ -26,7 +26,7 @@
 </script>
 <!-- DemoGrid component script -->
 <script>
-const { reactive, computed, toRefs } = Vue
+const { reactive, computed } = Vue
 
 const capitalize = str => str.charAt(0).toUpperCase() + str.slice(1)