Workaround for Relish

This is another fix for Relish rendering of trema/trema#258.

It looks like in order to control the order of groups in .nav
following conditions must be met. 

* Groups should not be nested
* Group name(=directory name) should not contain '-'

Relish output will look like this:
https://www.relishapp.com/y-higuchi/trema-testbed/docs