GridFS docs fixes.
GridFS docs fixes.

ShaneHarvey/mongo-python-driver,ramnes/mongo-python-driver,mongodb/mongo-python-driver,mongodb/mongo-python-driver,aherlihy/mongo-python-driver,ramnes/mongo-python-driver,ShaneHarvey/mongo-python-driver,ShaneHarvey/mongo-python-driver,ramnes/mongo-python-driver,mongodb/mongo-python-driver,aherlihy/mongo-python-driver,aherlihy/mongo-python-driver