Update introduction.md (#796)

* Update introduction.md

I was working through the examples in Vue 3 and got the following error:
```
Elements in iteration expect to have 'v-bind:key' directives  vue/require-v-for-key
```

Adding the key removes this error. I'm new to Vue (coming from React) and don't know if I'm missing something.

* Update src/guide/introduction.md

Co-authored-by: Natalia Tepluhina <[email protected]>

Author: doingandlearning

src/guide/introduction.md

@@ -189,7 +189,7 @@ There are quite a few other directives, each with its own special functionality.
 ```html
 <div id="list-rendering">
   <ol>
-    <li v-for="todo in todos">
+    <li v-for="todo in todos" v-bind:key="todo.text">
       {{ todo.text }}
     </li>
   </ol>