Quote version value in Docker Compose file

The example Docker Compose file does not quote the version value however
the Docker Compose documentation specifies this value should be quoted
to distinguish it from being treated as a number.

Relate elastic/elasticsearch#27745

Original commit: elastic/x-pack-elasticsearch@42ad68c3ac

docs/en/setup/docker.asciidoc

@@ -178,7 +178,7 @@ endif::[]
 ifeval::["{release-state}"!="unreleased"]
 ["source","yaml",subs="attributes"]
 --------------------------------------------
-version: 2.2
+version: '2.2'
 services:
   elasticsearch:
     image: {docker-image}