Slack integration - correct script location

[gaddman]

In the Slack example there's a mismatch between the documented script location and the example pantheon.yml file. The document describes using the private location:

Add, and update as needed, the example slack_notification.php script to the private directory in the root of your site's codebase, that is under version control.

The pantheon.yml file refers to private/scripts:

          script: private/scripts/slack_notification.php

For clarity it would be good to be explicit here, eg change the documentation to read:

Add, and update as needed, the example slack_notification.php script to the private/scripts directory in the root of your site's codebase, that is under version control.