fix: Add key to v-for (#1369)

Author: keng0o

examples/shopping-cart/components/ProductList.vue

@@ -1,6 +1,8 @@
 <template>
   <ul>
-    <li v-for="product in products">
+    <li
+      v-for="product in products"
+      :key="product.id">
       {{ product.title }} - {{ product.price | currency }}
       <br>
       <button

examples/shopping-cart/components/ShoppingCart.vue

@@ -3,7 +3,9 @@
     <h2>Your Cart</h2>
     <p v-show="!products.length"><i>Please add some products to cart.</i></p>
     <ul>
-      <li v-for="product in products">
+      <li
+        v-for="product in products"
+        :key="product.id">
         {{ product.title }} - {{ product.price | currency }} x {{ product.quantity }}
       </li>
     </ul>