Fix path for amazon-ssm-agent in base-ec2.ign

[a-wh]

Fix ExecStart path for amazon-ssm-agent in base-ec2.ign

The current ExecStart path for amazon-ssm-agent is incorrect. This PR fixes the path in the ignition config used to create the amazon-ssm-agent unit.

How to use

systemctl status amazon-ssm-agent

Testing done

# ls -lahrt /usr/share/oem/amazon-ssm-agent
ls: cannot access '/usr/share/oem/amazon-ssm-agent': No such file or directory

# ls -lahrt /usr/share/oem/bin/amazon-ssm-agent
-rwxr-xr-x. 1 root root 31M Aug 18 01:28 /usr/share/oem/bin/amazon-ssm-agent

     Loaded: loaded (/etc/systemd/system/amazon-ssm-agent.service; enabled; vendor preset: enabled)
     Active: active (running) since Wed 2021-08-25 14:40:52 UTC; 1 day 18h ago
   Main PID: 1256 (amazon-ssm-agen)
      Tasks: 9 (limit: 3419)
     Memory: 25.2M
        CPU: 514ms
     CGroup: /system.slice/amazon-ssm-agent.service
             └─1256 /usr/share/oem/bin/amazon-ssm-agent```

[jepio]

Thanks, it's great to see contributors fix these kinds of issues.