nested <h2> elements break table of contents

[trebor]

if a markdown page includes content like:

## Section Three

<h2>Lorem ipsum</h2>

it will cause the table of contents cursor to skip past the Section Three entry.

this is true even if the <h2> is embedded more deeply in html.

## Section Three

<div class="card">
  <h2>Lorem ipsum</h2>
</div>

this bug can be seen in the current CLI documentation for cards:

https://cli.observablehq.com/layout/card