Update googlecompute.markdown

A very small hint that you can't just use "packer-images", since buckets need to be globally unique, and must already exist. Attempting to use that results in the error 'Got status 403 from attempt to start resumable upload. Aborting', 'ABORT'

website/source/docs/builders/googlecompute.markdown

@@ -55,7 +55,7 @@ files obtained in the previous section.
 <pre class="prettyprint">
 {
   "type": "googlecompute",
-  "bucket_name": "packer-images",
+  "bucket_name": "my-project-packer-images",
   "client_secrets_file": "client_secret.json",
   "private_key_file": "XXXXXX-privatekey.p12",
   "project_id": "my-project",
@@ -72,7 +72,7 @@ each category, the available options are alphabetized and described.
 Required:
 
 * `bucket_name` (string) - The Google Cloud Storage bucket to store the
-  images that are created.
+  images that are created. The bucket must already exist in your project.
 
 * `client_secrets_file` (string) - The client secrets JSON file that
   was set up in the section above.