manual: Update folder sharing instruction
manual: Update folder sharing instruction

Link to the folder with latest releases and remove outdate info about
the service registration.

Acked-by: Victor Toso <39b7599ff73bbf5db531748ba5250f21fa7120ff@redhat.com>

fgouget/spice,fgouget/spice,fgouget/spice,fgouget/spice