Update sample code snippet in the docs

Fixes elastic/docs-content#3368

Author: theletterf

docs/index.asciidoc

@@ -147,7 +147,7 @@ You can customise the line format using the `line` codec like:
 [source,ruby]
 -----
 output {
- file {
+ csv {
    path => ...
    codec => line { format => "custom format: %{message}"}
  }
@@ -180,4 +180,4 @@ may not make the values safe in your spreadsheet application
 include::{include_path}/{type}.asciidoc[]
 
 
-:default_codec!:
+:default_codec!: