Fix developer guide TOC
Fix developer guide TOC

The table of contents for our developer guide does not show due to the
fact that it is past the first sub-header.

Change-Id: I8459a4949e3e4822b0a3cd4f163475d2c60b0f2e

switch-ch/diskimage-builder,zioproto/diskimage-builder,openstack/diskimage-builder,switch-ch/diskimage-builder,zioproto/diskimage-builder,openstack/diskimage-builder