chore: return product id in shopping cart example (#1918)

fix #1918

Author: kiaking

examples/shopping-cart/components/ShoppingCart.vue

@@ -1,11 +1,11 @@
 <template>
   <div class="cart">
     <h2>Your Cart</h2>
-    <p v-show="!products.length"><i>Please add some products to cart.</i></p>
+    <p v-show="!products.length">
+      <i>Please add some products to cart.</i>
+    </p>
     <ul>
-      <li
-        v-for="product in products"
-        :key="product.id">
+      <li v-for="product in products" :key="product.id">
         {{ product.title }} - {{ product.price | currency }} x {{ product.quantity }}
       </li>
     </ul>

examples/shopping-cart/store/modules/cart.js

@@ -13,6 +13,7 @@ const getters = {
     return state.items.map(({ id, quantity }) => {
       const product = rootState.products.all.find(product => product.id === id)
       return {
+        id: product.id,
         title: product.title,
         price: product.price,
         quantity