update custom directive section
update custom directive section

NOTE:
  pick up from https://github.com/vuejs/vuejs.org/commit/ed6a524151f1f85c078e0616d4bbe49d7d0f619b

potato4d/jp.vuejs.org,vuejs/jp.vuejs.org,potato4d/jp.vuejs.org,vuejs/jp.vuejs.org