DOC: minor typo + more adequate exception message about SUBJECTS_DIR
DOC: minor typo + more adequate exception message about SUBJECTS_DIR

Eric89GXL/PySurfer,effigies/PySurfer,haribharadwaj/PySurfer,mwaskom/PySurfer,diego0020/PySurfer,nipy/PySurfer,bpinsard/PySurfer