primeira-app-v2.html

<script src="https://cdn.jsdelivr.net/npm/vue/dist/vue.js"></script>

<div id="app">
    <input type="text" v-on:input="alterTitle">
    <p>{{ title }}</p>
</div>

<script>
    new Vue({
        el: '#app',
        data: {
            title: 'Hello World Vue!',
        },
        methods: {
            alterTitle(event) {
                this.title = event.target.value; 
            }
        }
    })
</script>