Fix bbregister init-reg argument

Author: juhuntenburg

nipype/interfaces/freesurfer/preprocess.py

@@ -807,7 +807,7 @@ class BBRegisterInputSpec(FSTraitedSpec):
     init = traits.Enum('spm', 'fsl', 'header', argstr='--init-%s',
                        mandatory=True, xor=['init_reg_file'],
                        desc='initialize registration spm, fsl, header')
-    init_reg_file = File(exists=True,
+    init_reg_file = File(exists=True, argstr='--init-reg %s',
                          desc='existing registration file',
                          xor=['init'], mandatory=True)
     contrast_type = traits.Enum('t1', 't2', argstr='--%s',