add v-if to the contact details

cmp-intro-01-starting-setup/index.html

@@ -19,8 +19,8 @@
         <ul>
             <li v-for="friend in friends">
                 <h2>{{ friend.name }}</h2>
-                <button>Show Details</button>
-                <ul>
+                <button @click="toggleDetails()">Show Details</button>
+                <ul v-if="detailsAreVisible">
                     <li><strong>Phone:</strong> {{ friend.phone }}</li>
                     <li><strong>Email:</strong> {{ friend.email }}</li>
                 </ul>