Different spacing above and below code blocks

This is a visual bug: when it’s enabled, the spacing above code blocks is larger than the one below them. Screenshot:
<img width="400" alt="圖片" src="https://user-images.githubusercontent.com/3153452/109409808-89b7d880-794a-11eb-8b3b-54a24bb6a52c.png">

A possible fix is to change `<div class="codecopy">` into a `<p>`, to make use of the existing CSS rules.