[doc] incus list documentation formatting

[bensmrs]

https://linuxcontainers.org/incus/docs/main/reference/manpages/incus/list/, generated from doc/reference/manpages/incus/list.md, in turn generated from the manpages, is wrongly formatted. Additional processing should be done on the command description.

[stgraber]

Hmm, we basically entirely rely on spf13/cobra for that stuff. Using the markdown output from incus manpage PATH --format=md --all, so we'd need to try and tweak the input to get a more suitable output.