Fix NameFormat documentation: change `-` to `_`

As pointed out by @roadhump, the default value for the `convention`
option is `hyphenated_lowercase`, not the `hyphenated-lowercase` that
appeared in the documentation.

Fixes #147

Change-Id: I60d86e728dbff06997e47c909629948bd69743af
Reviewed-on: http://gerrit.causes.com/37554
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Joe Lencioni <joe.lencioni@causes.com>