The first paragraph on page 219 mentions a checkbox:
Then, we use the
heading/1component that we’ll define in a bit to show a heading and a checkbox if the ratings are complete.
The next paragraph describes that symbol as a checkmark:
… we render the unicode for a checkmark to indicate to the user that all of the ratings forms have been completed.
The heading function uses Unicode U+2713, the checkmark.
<h2 class="font-medium text-2xl">
Ratings <%= if ratings_complete?(@products), do: raw "✓" %>
</h2>