doc: fix typo in vmsingle k8s monitoring guide (#8120)

As reported by a user, the value in [VictoriaMetrics Single helm
chart](https://github.com/VictoriaMetrics/helm-charts/blob/master/charts/victoria-metrics-single/values.yaml#L420)
and actual example, should be "enabled", not "enable". This commit fixes
it.


### Describe Your Changes

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### Checklist

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- [x] My change adheres [VictoriaMetrics contributing
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docs/guides/k8s-monitoring-via-vm-single/README.md

@@ -148,7 +148,7 @@ server:
 
 
 * By running `helm install vmsingle vm/victoria-metrics-single` we install [VictoriaMetrics Single](https://docs.victoriametrics.com/single-server-victoriametrics/) to default [namespace](https://kubernetes.io/docs/concepts/overview/working-with-objects/namespaces/) inside your cluster
-* By adding `scrape: enable: true` we add and enable autodiscovery scraping from kubernetes cluster to [VictoriaMetrics Single](https://docs.victoriametrics.com/single-server-victoriametrics/)
+* By adding `scrape: enabled: true` we add and enable autodiscovery scraping from kubernetes cluster to [VictoriaMetrics Single](https://docs.victoriametrics.com/single-server-victoriametrics/)
 * On line 166 from [https://docs.victoriametrics.com/guides/examples/guide-vmsingle-values.yaml](https://docs.victoriametrics.com/guides/examples/guide-vmsingle-values.yaml) we added `metric_relabel_configs` section that will help us to show Kubernetes metrics on Grafana dashboard.