Update cpac_pipeline.py

Fixed the fnirt_config input parameter to pass the correct value (as per updated config.yml file).

fnirt_config input of reg_anat_mni only accepts the name of the FSL config file, not the entire path. As per the Nipype documentation:

FSL FNIRT > Inputs > Optional

"config_file: ('T1_2_MNI152_2mm' or 'FA_2_FMRIB58_1mm' or an existing file name)
        Name of config file specifying command line arguments"