message: build(aio): improve rendering of directive selectors
diff: {%- if doc.selector %}
  <section class="selectors">
  <h2>Selectors</h2>
  - {% for selector in doc.selector.split(',') %}
  - <div class="selector">
  - <code>{$ selector $}</code>
  - </div>
  - {% endfor %}
  + <code-example hideCopy="true" class="no-box api-heading selector">
  + {%- for selector in doc.selector.split(',') %}
  + {$ selector $}{% endfor %}
  + </code-example>
  </section>
  {% endif %}
  