Update nbgrader/api.py
Update nbgrader/api.py

according to jhamrick suggestion

Co-Authored-By: danielmaitre <8a0a532bd8e75d52462030a27ba3b5977dde83fc@durham.ac.uk>
jupyter/nbgrader,jupyter/nbgrader,jupyter/nbgrader,jupyter/nbgrader,jhamrick/nbgrader,jupyter/nbgrader,jhamrick/nbgrader,jhamrick/nbgrader,jhamrick/nbgrader