Move example plugin to the custom plugins directory.

I know there's some contention over whether the `custom` directory should exist at all, but while it still does, we may as well structure the example correctly.  This confused the heck out of me until I noticed that plugins were supposed to go in `custom/plugins`, and not just `custom` like the example.

I was surprised not to find an existing pull request for this.  Maybe I just didn't search well enough.