amazon-ebs update simple example

update simple example to make it copy/paste works

website/source/docs/builders/amazon-ebs.html.md

@@ -549,14 +549,20 @@ run:
 
 ``` json
 {
-  "type": "amazon-ebs",
-  "access_key": "YOUR KEY HERE",
-  "secret_key": "YOUR SECRET KEY HERE",
-  "region": "us-east-1",
-  "source_ami": "ami-fce3c696",
-  "instance_type": "t2.micro",
-  "ssh_username": "ubuntu",
-  "ami_name": "packer-quick-start {{timestamp}}"
+    "variables": {
+        "aws_access_key": "{{env `AWS_ACCESS_KEY_ID`}}",
+        "aws_secret_access_key": "{{env `AWS_SECRET_ACCESS_KEY`}}"
+    },
+    {
+        "type": "amazon-ebs",
+        "access_key": "{{user `aws_access_key`}}",
+        "secret_key": "{{user `aws_secret_access_key`}}",
+        "region": "us-east-1",
+        "source_ami": "ami-fce3c696",
+        "instance_type": "t2.micro",
+        "ssh_username": "ubuntu",
+        "ami_name": "packer-quick-start {{timestamp}}"
+    }
 }
 ```