api: change <pk> parameter to <cover_id> for cover comments endpoint

api: change <pk> parameter to <cover_id> for cover comments endpoint

Rename cover lookup parameter `pk` to `cover_id` for the cover comments
list endpoints to disambiguate from the lookup parameter `comment_id` in
upcoming patches which introduces the cover comments detail endpoint.
This doesn't affect the user-facing API.

Signed-off-by: Raxel Gutierrez <raxel@google.com>
Reviewed-by: Stephen Finucane <stephen@that.guru>
[dja: fixup to apply on top of fecf7c86c2c5 and 0ed2feb26c84]
Signed-off-by: Daniel Axtens <dja@axtens.net>