Use underscores instead of dashes in gitlab.rb

[ci skip]

doc/administration/monitoring/prometheus/postgres_exporter.md

@@ -13,7 +13,7 @@ To enable the postgres exporter:
 1. Add or find and uncomment the following line, making sure it's set to `true`:
 
     ```ruby
-    postgres-exporter['enable'] = true
+    postgres_exporter['enable'] = true
     ```
 
 1. Save the file and [reconfigure GitLab][reconfigure] for the changes to

doc/administration/monitoring/prometheus/redis_exporter.md

@@ -14,7 +14,7 @@ To enable the Redis exporter:
 1. Find and uncomment the following line, making sure it's set to `true`:
 
     ```ruby
-    redis-exporter['enable'] = true
+    redis_exporter['enable'] = true
     ```
 
 1. Save the file and [reconfigure GitLab][reconfigure] for the changes to