Update Cookbook ssh_agent, service name and command

[wanesty]

forgot the service name when i first wrote allat :p

[wanesty]

i was debating whether or not to add the systemctl command and i initially didn't on the first PR because linux users using a systemd distro should know that, which follows the arch wiki standard of linking people to the page teaching them how to do something they're gonna do a lot instead of just brainlessly giving them the command.
but being able to quickly grab the command is useful in this context, like this is admittedly mostly for myself but it can help others.