Merge branch 'fix-prometheus-docs-custom-query' into 'master'

Fix Prometheus custom query variable docs

Closes #32775

See merge request gitlab-org/gitlab!19178

doc/user/project/integrations/prometheus.md

@@ -139,7 +139,7 @@ GitLab supports a limited set of [CI variables](../../../ci/variables/README.htm
 - CI_ENVIRONMENT_SLUG
 - KUBE_NAMESPACE
 
-To specify a variable in a query, enclose it in curly braces with a leading percent. For example: `%{ci_environment_slug}`.
+To specify a variable in a query, enclose it in quotation marks with curly braces with a leading percent. For example: `"%{ci_environment_slug}"`.
 
 ### Defining custom dashboards per project