netlink: specs: add the multicast group name to spec

Add the multicast group's name to the YAML spec.
Without it YNL doesn't know how to subscribe to notifications.

Reviewed-by: Donald Hunter <donald.hunter@gmail.com>
Link: https://patch.msgid.link/20250623231720.3124717-2-kuba@kernel.org


Signed-off-by: Jakub Kicinski <kuba@kernel.org>