fix. promtheus link error

Signed-off-by: Colvin-Y <[email protected]>

Author: Colvin-Y

content/en/docs/concepts/cluster-administration/system-metrics.md

@@ -250,7 +250,7 @@ is encountered that is not allowed with respect to the allow-list constraints.
 
 ## {{% heading "whatsnext" %}}
 
-* Read about the [Prometheus text format](https://github.com/prometheus/docs/blob/master/content/docs/instrumenting/exposition_formats.md#text-based-format)
+* Read about the [Prometheus text format](https://github.com/prometheus/docs/blob/main/docs/instrumenting/exposition_formats.md#text-based-format)
   for metrics
 * See the list of [stable Kubernetes metrics](https://github.com/kubernetes/kubernetes/blob/master/test/instrumentation/testdata/stable-metrics-list.yaml)
 * Read about the [Kubernetes deprecation policy](/docs/reference/using-api/deprecation-policy/#deprecating-a-feature-or-behavior)