Increase restart delay for systemd service

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Redshift just crashed for me, presumably because the default RestartSec (100ms) is too short:

Oct 06 11:47:03 pavilion-arch systemd[473]: redshift.service: Scheduled restart job, restart counter is at 5.
Oct 06 11:47:03 pavilion-arch systemd[473]: Stopped Redshift display colour temperature adjustment.
Oct 06 11:47:03 pavilion-arch systemd[473]: redshift.service: Start request repeated too quickly.
Oct 06 11:47:03 pavilion-arch systemd[473]: redshift.service: Failed with result 'exit-code'.
Oct 06 11:47:03 pavilion-arch systemd[473]: Failed to start Redshift display colour temperature adjustment.