Changes to rails system tests guidelines. Update sudo command

[Disco-Chef]

With the current install instructions of rails Stupid Coaching, I had some windows students getting:
E: Unable to locate package google-chrome-stable_current_amd64.deb

Then it hit me: command should be sudo apt install ./google-chrome-stable_current_amd64.deb if using apt with a path to a deb file and not fetching by package name from the Ubuntu repositories.

All three had to also then sudo apt --fix-broken install for some reason 🤷‍♂️