docs: netlink: clean up after deprecating version

Jiri moved version to legacy specs in commit 0f07415e ("netlink:
specs: don't allow version to be specified for genetlink").
Update the documentation.

Reviewed-by: Jiri Pirko <jiri@nvidia.com>
Link: https://lore.kernel.org/r/20231016214540.1822392-1-kuba@kernel.org


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