Fix relative path between sphinx conf.py file and Nic table file
to allow automatic build on ReadTheDocs.
Fixes: 9db3f52126fb ("doc: generate NIC overview table from ini files")
Signed-off-by: John McNamara <john.mcnamara@intel.com>
Tested-by: Ferruh Yigit <ferruh.yigit@intel.com>
Acked-by: Pablo de Lara <pablo.de.lara.guarch@intel.com>