[FIX] Fix plot_fsaverage path finding

If FreeSurfer isn't installed will download requested surface to
~/nnt-data/tpl-fsaverage and use that, instead, but the paths weren't
being set up correctly. This should resolve that issue

Author: rmarkello

netneurotools/plotting.py

@@ -387,7 +387,7 @@ def plot_fsaverage(data, *, lhannot, rhannot, order='LR', surf='pial',
         from .datasets import fetch_fsaverage
         from .datasets.utils import _get_data_dir
         fetch_fsaverage()
-        subjects_dir = _get_data_dir()
+        subjects_dir = os.path.join(_get_data_dir(), 'tpl-fsaverage')
         subject_id, subjects_dir = check_fs_subjid(subject, subjects_dir)
 
     # cast data to float (required for NaNs)