Fix obmc-fru-fault-monitor.service ExecStart path

A previous patch introduced a bug in the obmc-fru-fault-monitor
service file where the ExecStart path was missing a leading
slash. As a result, systemd failed to locate the executable.

This patch corrects the path by adding the missing slash.

Tested:
Checked that the service is correctly loaded, and
that the binary is present:
```
root@bmc:/usr/libexec/phosphor-led-manager# systemctl status obmc-fru-fault-monitor.service
* obmc-fru-fault-monitor.service - FRU Fault monitor service
     Loaded: loaded (/usr/lib/systemd/system/obmc-fru-fault-monitor.service; disabled; preset: disabled)
     Active: inactive (dead)
root@bmc:/usr/libexec/phosphor-led-manager# ls | grep fru
phosphor-fru-fault-monitor
```

Change-Id: I07e1558b7e3cf5b5517c1c4c075ea15d67a4f7f6
Signed-off-by: William de Abreu Pinho <williamdapinho@gmail.com>