Update v-model.md (vuejs#2890)

* Update v-model.md

> When it is mutated by the child, it causes the parent bound value to be updated as well.

Maybe adding a click event to invoke the update would help clarify the example?

* Update src/guide/components/v-model.md

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

---------

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

Author: Yordan-Ramchev

src/guide/components/v-model.md

@@ -20,6 +20,7 @@ function update() {
 
 <template>
   <div>Parent bound v-model is: {{ model }}</div>
+  <button @click="update">Increment</button>
 </template>
 ```