Merge pull request #3583 from justinsb/docs_custom_metrics_2

Automatic merge from submit-queue.

Docs tweak - specify flag, not kube-up env var

docs/cluster_spec.md

@@ -195,14 +195,15 @@ NOTE: Where the corresponding configuration value can be empty, fields can be se
 Will result in the flag `--resolv-conf=` being built.
 
 #### Enable Custom metrics support
-To use custom metrics in kubernetes as per [custom metrics doc](https://kubernetes.io/docs/tasks/run-application/horizontal-pod-autoscale/#support-for-custom-metrics) we have to set flag  `ENABLE_CUSTOM_METRICS` to `true` on all the kubelets.
-we can specify that in the `kubelet` spec in our cluster.yml.
+To use custom metrics in kubernetes as per [custom metrics doc](https://kubernetes.io/docs/tasks/run-application/horizontal-pod-autoscale/#support-for-custom-metrics)
+we have to set the flag `--enable-custom-metrics` to `true` on all the kubelets. We can specify that in the `kubelet` spec in our cluster.yml.
 
 ```
 spec:
   kubelet:
     enableCustomMetrics: true
 ```
+
 ### kubeScheduler
 
 This block contains configurations for `kube-scheduler`.  See https://kubernetes.io/docs/admin/kube-scheduler/