DOC: Fix build of FreeSurfer workflow graph

Author: effigies

niworkflows/anat/freesurfer.py

@@ -36,7 +36,7 @@ def init_gifti_surface_wf(
             :simple_form: yes
 
             from niworkflows.anat.freesurfer import init_gifti_surface_wf
-            wf = init_gifti_surface_wf()
+            wf = init_gifti_surface_wf(subjects_dir="/tmp")
 
     Parameters
     ----------