References: The `section-desc` class isn't being applied the description

[gadenbuie]

Using pkgdown's own site for a reprex, the Build section of the reference index includes a description:

reference:
- title: Build
  desc:  Build a complete site or one of its components.

But in the rendered HTML, the section-desc class is being applied to an empty <p> tag, rather than the description.

<div class="section level2">
  <h2 id="build">Build<a class="anchor" aria-label="anchor" href="#build"></a></h2>
  <p class="section-desc"></p>
  <p>Build a complete site or one of its components.</p>
</div>