fix: sigma units

Author: satra

nipype/interfaces/ants/registration.py

@@ -394,7 +394,7 @@ def _formatRegistration(self):
             retval.append('--convergence %s' % self._formatConvergence(ii))
             retval.append('--smoothing-sigmas %s%s' % (self._antsJoinList(
                 self.inputs.smoothing_sigmas[ii]),
-                          self.inputs.sigma_units))
+                          self.inputs.sigma_units[ii]))
             retval.append('--shrink-factors %s' %
                           self._antsJoinList(self.inputs.shrink_factors[ii]))
             if isdefined(self.inputs.use_estimate_learning_rate_once):