ye.. maybe should write the "Chain Rule":

dy/dx =dy/du * du/dx

in this case ln(1-x):
y=ln(1-x)
u=1-x

dy/du=d (lnu)/du=1/u
du/dx=-1
=>
dy/dx = dy/du * du/dx = 1/u * (-1) = -1/(1-x)