fix(docs): Modify GCP role/objectViewer -> role/storage.objectViewer

closes spinnaker#204

content/en/docs/setup/install/providers/docker-registry.md

@@ -103,7 +103,7 @@ API](https://console.developers.google.com/apis/api/cloudresourcemanager.googlea
  A [service account](https://cloud.google.com/compute/docs/access/service-accounts)
  is the preferred way to authenticate to GCR. Use the commands below to create
  and download a service account to be used as your password with the required
- `roles/objectViewer` role, assuming the registry exists in your current
+ `roles/storage.objectViewer` role, assuming the registry exists in your current
  `gcloud` project.
 
  (You can use an [access
@@ -131,7 +131,7 @@ API](https://console.developers.google.com/apis/api/cloudresourcemanager.googlea
 
  gcloud projects add-iam-policy-binding $PROJECT \
  --member serviceAccount:$SA_EMAIL \
- --role roles/objectViewer
+ --role roles/storage.objectViewer
 
  mkdir -p $(dirname $SERVICE_ACCOUNT_DEST)