Update README.md

Author: David-Desmaisons

README.md

@@ -39,7 +39,7 @@ Use draggable component:
 ### Typical use:
 ``` html
 <draggable v-model="myArray" :options="{group:'people'}" @start="drag=true" @end="drag=false">
-   <div v-for="element in myArray">{{element.name}}</div>
+   <div v-for="element in myArray" :key="element.id">{{element.name}}</div>
 </draggable>
 ```
 .vue file: