Change SSH protocol domain (update arising from registry)

I have been drafting wording for an ssh protocol assigned names registry, apropos #1049 (closed). (I will make an MR against torspec shortly.) This prompted me to consider the Arti specifications for SSH keys.

I propose the following change in the Arti codebase:

Use @spec.torproject.org for ssh key extension algorithms, not @torproject.org. Experience from PuTTY (whose maintainer is a friend of mine) suggests that the resemblance to email addresses is confusing. It's not desirable for the namespace to overlap with things that are or might become email addresses.