Change amazon.ami to amazon.image.

... to be consistent with https://github.com/hashicorp/terraform/pull/3195.

website/source/docs/post-processors/atlas.html.markdown

@@ -107,7 +107,7 @@ you can also use `token` configuration option.
         "type": "atlas",
         "token": "{{user `atlas_token`}}",
         "artifact": "hashicorp/foobar",
-        "artifact_type": "amazon.ami",
+        "artifact_type": "amazon.image",
         "metadata": {
           "created_at": "{{timestamp}}"
         }

website/source/intro/getting-started/remote-builds.html.markdown

@@ -99,7 +99,7 @@ deployed by a tool like [Terraform](https://terraform.io). The `atlas`
   "post-processors": [{
     "type": "atlas",
     "artifact": "ATLAS_USERNAME/packer-tutorial",
-    "artifact_type": "amazon.ami"
+    "artifact_type": "amazon.image"
   }]
 }
 ```