Adding OpenGL/GPU support to a snap

I wish to report something confusing (for newbies like me):

this page tells: ’ The next step is to copy the desktop-glib-only part from the desktop helpers snapcraft.yaml file.’

but the link to the other page does not contain the exact code: ‘source: .’ instead of ‘source: GitHub - ubuntu/snapcraft-desktop-helpers: Various launchers for snapcraft wiki parts

This leads to the following error when building:

$snapcraft --verbose
...
/root/parts/desktop-glib-only/build/glib-only: No such file or directory                                                                        
Failed to execute pack in instance.