Use new prometheus metrics URL to avoid an extra redirection

Signed-off-by: Takuya Noguchi <takninnovationresearch@gmail.com>

doc/user/project/integrations/prometheus.md

@@ -296,7 +296,7 @@ Data from Prometheus charts on the metrics dashboard can be downloaded as CSV.
 
 #### Managed Prometheus instances
 
-> [Introduced](https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/6590) in [GitLab Ultimate](https://about.gitlab.com/pricing/) 11.2 for [custom metrics](#adding-additional-metrics-premium), and 11.3 for [library metrics](prometheus_library/metrics.md).
+> [Introduced](https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/6590) in [GitLab Ultimate](https://about.gitlab.com/pricing/) 11.2 for [custom metrics](#adding-additional-metrics-premium), and 11.3 for [library metrics](prometheus_library/index.md).
 
 For managed Prometheus instances using auto configuration, alerts for metrics [can be configured](#adding-additional-metrics-premium) directly in the performance dashboard.