Thanks for your reply Joseph.
That language editing doc is not quite accurate for me so I'll put my method here in case anyone with the same problems stumbles across this post.
My problem was I do not have a Language Editing option as the doc says. I had to go to Site Administration > Language > Language Customization. I then got a dropdown menu with English as the only option. After selecting that I got a button that says Check out String into Translator. Pressing that brought up "Strings checked out successfully into the translator" and a green bar with a continue button underneath. Clicking that brings up the option to find and alter strings. The one I wanted was under Mod, lesson.php as Joseph said.