Fix doc autoindexing.

The config section get returns a tuple, not a string, so we were trying
to lower() a tuple. That clearly does not work.

Change-Id: I96602a25686d447bd3437807d9e55aaec6bb9540
Reviewed-on: https://review.openstack.org/30520
Reviewed-by: Chris Krelle <nobodycam@gmail.com>
Reviewed-by: Devananda van der Veen <devananda.vdv@gmail.com>
Reviewed-by: Clark Boylan <clark.boylan@gmail.com>
Reviewed-by: James E. Blair <corvus@inaugust.com>
Approved: James E. Blair <corvus@inaugust.com>
Tested-by: Jenkins