Fix whitelist docs.

At the moment this links to [a file](https://github.com/pjcdawkins/platformsh-examples/blob/default-whitelist/.platform.app.yaml#L25) in my fork of `platformsh/platformsh-examples` - that file should be reviewed in this PR. I'm not sure how many keys are required in that `.platform.app.yaml` file, and which have default fallbacks...