dev_guide: Add "sudo"; don't use "-proposed" package repository
dev_guide: Add "sudo"; don't use "-proposed" package repository

When adding the Ubuntu Debug Symbol Packages repository, instructions
tell us to add the "-proposed" repo, but this is a repository that
contains pre-release packages for the next Ubuntu release, so we don't
really want it in our systems.

See:
- https://wiki.ubuntu.com/Debug_Symbol_Packages
- https://wiki.ubuntu.com/Testing/EnableProposed

Kurento/doc-kurento,Kurento/doc-kurento,Kurento/doc-kurento