fix reference error (vuejs#673)

Author: kazupon

src/v2/guide/components.md

@@ -629,7 +629,10 @@ Vue.component('currency-input', {
     }
   }
 })
-new Vue({ el: '#currency-input-example' })
+new Vue({
+  el: '#currency-input-example',
+  data: { price: '' }
+})
 </script>
 {% endraw %}