docs: use local icon to prevent cors issues on stackblitz

Author: danielroe

playground/pages/index.vue

@@ -3,7 +3,7 @@
  <ion-headertranslucent>
  <ion-toolbarclass="toolbar">
  <ion-thumbnailslot="start">
- <ion-imgsrc="https://nuxtjs.org/design-kit/colored-logo.svg" />
+ <ion-imgsrc="/icon.png" />
  </ion-thumbnail>
  <ion-title> Nuxt Ionic </ion-title>
  </ion-toolbar>