bug#39700: Outdated installation video

[Julien Lepiller]

Hi guix!

This email is motivated by https://social.tchncs.de/@mray/103492792270770063 
who complained that the instructions didn't work (not in a very nice way :/).

In fact, the video is slightly outdated, showing a different command for 
importing the gpg key.

There was also a confusion as the gpg fingerprint appears on a separate 
paragraph, as if it were part of the output, leading to a failure due to the 
lack of providing a fingerprint. This points to an issue in the video 
generation script.

Anyway, this long fingerprint is not necessary anymore, since the script now 
suggests to import from savannah. My patch from yesterday also proposes to add 
an option that is easier to type to let the script do it. We should update the 
video accordingly.

Thanks :)