Update stressng.md to reflect crd requirements (#825)

```
looking for "jobfile.j2" at "/opt/ansible/roles/stressng/templates/jobfile.j2"
File lookup using /opt/ansible/roles/stressng/templates/jobfile.j2 as file
fatal: [localhost]: FAILED! => {
    "msg": "The task includes an option with an undefined variable. The error was: 'dict object' has no attribute 'cpu_method'. 'dict object' has no attribute 'cpu_method'. 'dict object' has no attribute 'cpu_method'. 'dict object' has no attribute 'cpu_method'\n\nThe error appears to be in '/opt/ansible/roles/stressng/tasks/main.yaml': line 4, column 5, but may\nbe elsewhere in the file depending on the exact syntax problem.\n\nThe offending line appears to be:\n\n\n  - name: template stressng config file\n    ^ here\n"
}
```

docs/stressng.md

@@ -39,6 +39,7 @@ spec:
       # cpu stressor options
       cpu_stressors: "1"
       cpu_percentage: "100"
+      cpu_method: "all"
       # vm stressor option
       vm_stressors: "1"
       vm_bytes: "128M"