Fix configuration display

[Jonathan Petitcolas]

From #198:

As is, some parts of the configuration guide is pretty hard to understand. Since every heading is an h3 element, it's not clear which should be attributes of the config object and which should be sub-attributes. The sections that include examples are much more clear.

And that's true: https://github.com/marmelab/EventDrops/blob/master/docs/configuration.md