add missing reference for FSL flirt
add missing reference for FSL flirt

poldracklab/fmriprep,poldracklab/fmriprep,poldracklab/preprocessing-workflow,oesteban/fmriprep,oesteban/preprocessing-workflow,poldracklab/fmriprep,poldracklab/preprocessing-workflow,oesteban/fmriprep,oesteban/preprocessing-workflow,oesteban/fmriprep