Update systemd unit example with restart settings

[maxfischer2781]

In practice the COBalD unit must handle spurious failures gracefully. The unit file example should be updated with recommended settings for restarting.

For example, in production we now use the following:

[Unit]
StartLimitBurst=6
StartLimitIntervalSec=1h

[Service]
Restart=on-failure
RestartSec=300s