Modify docs to remove reference to Liquid docs

doc/user/project/integrations/prometheus.md

@@ -194,7 +194,7 @@ Variables for Prometheus queries must be lowercase.
 
 There are 2 methods to specify a variable in a query or dashboard:
 
-1. Variables can be specified using the [Liquid template format](https://shopify.dev/docs/liquid/reference/basics), for example `{{ci_environment_slug}}` ([added](https://gitlab.com/gitlab-org/gitlab/-/merge_requests/20793) in GitLab 12.6).
+1. Variables can be specified using double curly braces, such as `{{ci_environment_slug}}` ([added](https://gitlab.com/gitlab-org/gitlab/-/merge_requests/20793) in GitLab 12.7).
 1. You can also enclose it in quotation marks with curly braces with a leading percent, for example `"%{ci_environment_slug}"`. This method is deprecated though and support will be [removed in the next major release](https://gitlab.com/gitlab-org/gitlab/issues/37990).
 
 #### Editing additional metrics from the dashboard