Programming Phoenix 1.4 - updating nav in layout should be in root.html.heex? (page 97)

It says to update the nav section in the layout in ‘/lib/rumbl_web/templates/layout/app.html.eex’. I have 3 files in that directory and they are all .heex files. ‘app.html.heex’ does not have an existing nav section. There is a nav in ‘root.html.heex’ and when I implement the changes there, it all seems to work.