add missing arg

Author: josephmje

dmriprep/workflows/dwi/base.py

@@ -59,7 +59,7 @@ def init_dwi_preproc_wf(
                 omp_nthreads=1,
                 output_dir='.',
                 reportlets_dir='.',
-                use_syn,
+                use_syn=True,
                 layout=BIDSLayout('.'),
                 num_dwi=1,
             )