diff --git a/docs/index.asciidoc b/docs/index.asciidoc
index 7760d91..1ac7aa4 100644
--- a/docs/index.asciidoc
+++ b/docs/index.asciidoc
@@ -48,6 +48,7 @@ particular task. The name of this field is stored in `unique_id_field`.
 You can use a Grok filter to prepare the events for the elapsed filter.
 An example of configuration can be:
 [source,ruby]
+--------------------------------------------------
     filter {
       grok {
         match => { "message" => "%{TIMESTAMP_ISO8601} START id: (?<task_id>.*)" }
@@ -65,6 +66,7 @@ An example of configuration can be:
         unique_id_field => "task_id"
       }
     }
+--------------------------------------------------
 
 The elapsed filter collects all the "start events". If two, or more, "start
 events" have the same ID, only the first one is recorded, the others are
@@ -177,4 +179,4 @@ the last one will be used.
 
 
 [id="plugins-{type}s-{plugin}-common-options"]
-include::{include_path}/{type}.asciidoc[]
\ No newline at end of file
+include::{include_path}/{type}.asciidoc[]