The first paragraph on page 219 mentions a checkbox:

Then, we use the heading/1 component 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 "&#x2713;" %>
</h2>