[Bf-python] typo in epydocs

[jmsoler at free.fr]

However an object can belong to several layers,
so, why do not we simply write :

# to set
LAYER=4
OB.Layers |=2**(LAYER-1)

# to unset
LAYER=4
OB.Layers &=~2**(LAYER-1)

-jms-