Add override for NotificationARNs

This fixes a bug where the CLI argument was set to `--notification-ar-ns` ([docs](http://docs.aws.amazon.com/cli/latest/reference/cloudformation/update-stack.html)). The original parameter is `NotificationARNs`, as can be seen here:

http://docs.aws.amazon.com/AWSCloudFormation/latest/APIReference/API_UpdateStack.html

Confirmed to work in the CLI now:

```
SYNOPSIS
            update-stack
          --stack-name <value>
          [--template-body <value>]
          [--template-url <value>]
          [--use-previous-template | --no-use-previous-template]
          [--stack-policy-during-update-body <value>]
          [--stack-policy-during-update-url <value>]
          [--parameters <value>]
          [--capabilities <value>]
          [--stack-policy-body <value>]
          [--stack-policy-url <value>]
          [--notification-arns <value>]
```

@jamesls please review.