message: docs(alert): fix demo source code
diff: <ion-header translucent>
  <ion-toolbar>
  <ion-title>Alert</ion-title>
  - </ion-toolbar> </ion-header
  - >,
  + </ion-toolbar>
  + </ion-header>
  <ion-content fullscreen class="ion-padding">
  <ion-alert-controller></ion-alert-controller>
  <ion-button expand="block">Show Alert</ion-button>
  