updated include file to use code fences (#5229)

Signed-off-by: Victoria Bialas <victoria.bialas@docker.com>

_includes/content/compose-extfields-sub.md

@@ -2,6 +2,7 @@ It is possible to re-use configuration fragments using extension fields. Those
 special fields can be of any format as long as they are located at the root of
 your Compose file and their name start with the `x-` character sequence.
 
+```none
 version: '2.1'
 x-custom:
   items:
@@ -10,20 +11,24 @@ your Compose file and their name start with the `x-` character sequence.
   options:
     max-size: '12m'
   name: "custom"
+```
 
 The contents of those fields will be ignored by Compose, but they can be
 inserted in your resource definitions using [YAML anchors](http://www.yaml.org/spec/1.2/spec.html#id2765878).
 For example, if you want several of your services to use the same logging
 configuration:
 
+```none
 logging:
   options:
     max-size: '12m'
     max-file: 5
   driver: json-file
+```
 
 You may write your Compose file as follows:
 
+```none
 version: '2.1'
 x-logging:
   &default-logging
@@ -39,10 +44,12 @@ services:
   db:
     image: mysql:latest
     logging: *default-logging
+```
 
 It is also possible to partially override values in extension fields using
 the [YAML merge type](http://yaml.org/type/merge.html). For example:
 
+```none
 version: '2.1'
 x-volumes:
   &default-volume
@@ -61,3 +68,4 @@ the [YAML merge type](http://yaml.org/type/merge.html). For example:
     << : *default-volume
     driver: default
     name: volume-local
+```