update the name of the lxd-sphinx-extensions package

[ru-fu]

Using either lxd-sphinx-extensions or canonical-sphinx-extensions works, as well as including each extension with or without the canonical. prefix. But let's update it here to keep the starter pack as clean as possible.

[rkratky]

I just ran into a problem on https://github.com/canonical/ubuntu-packaging-guide where specifying youtube-links would cause an error (module not found). Changing it to canonical.youtube-links fixed it. I haven't investigated further yet. Just noting it, so that I don't forget :)