Revert "Fix azure image param"

This reverts commit 26f63d0.

Author: aktech

docs/reference/yaml.md

@@ -154,8 +154,11 @@ Machine image takes 4 parameters(publisher, offer, sku, version) for available i
 `az vm image list --output table`
 
 ```yml
-# publisher:offer:sku:version
-machine_image: Canonical:UbuntuServer:18.04-LTS:latest
+machine_image:
+  publisher: "Canonical"
+  offer: "UbuntuServer"
+  sku: "18.04-LTS"
+  version: "latest"
 ```
 
 #### Openstack