psu-monitor: adapt do chassis state instance change

phosphor-state-manager moved to a multi chassis and multi host design
point. This made their corresponding service files instance based. For
backwards support, instance 0 will always represent the entire system,
so utilize that as the dependency in this service file.

Tested:
- Verified in other similar commits that this type of relationship
  works as expected

Change-Id: If8ed9abcbb6752ed86c801755df18467cf1e9fe8
Signed-off-by: Andrew Geissler <geissonator@yahoo.com>
diff --git a/services/phosphor-psu-monitor.service b/services/phosphor-psu-monitor.service
index df896c5..fafcfbe 100644
--- a/services/phosphor-psu-monitor.service
+++ b/services/phosphor-psu-monitor.service
@@ -2,7 +2,7 @@
 Description=Phosphor Power Supply Monitor
 Wants=mapper-wait@-xyz-openbmc_project-inventory.service
 After=mapper-wait@-xyz-openbmc_project-inventory.service
-Before=xyz.openbmc_project.State.Chassis.service
+Before=xyz.openbmc_project.State.Chassis@0.service
 
 [Service]
 Restart=on-failure