Modern Front-End Development for Rails: B6 CSS data-day-toggle-hide-class in markup but not used PDF p 81

There is a data-day-toggle-hide-class in the section attributes in schedules#show that is never used in the DayToggleController and not explained in the text. Also the text tells you to modify the schedule day partial, but in the code, the section with the attributes is in show.html.erb right before the partial is rendered.

data-day-toggle-hide-class="hide"

The code works fine if you delete the attribute from the markup.