Mission15 : add ` gsh reset` reminder

[adomenec]

Hello,

I'm a teacher who's been using gameshell for a total of 8h (4 session of 2h each) with first year students. Each time, in the mission 15 (the mission asking to create and edit journal.txt with nano), I've got several student who create journal.txt using mkdir (because that's the only creation command they know), then ask me for help because nano can't edit folders.

I think that adding to that mission a reminder that gsh reset exists (or a reminder to gsh help) would be useful.

PS : much love for the awesome work that this project is !