The chapter 4 notebook, has this line:
matplotlib.rc('image', cmap='Greys')
it sets the default color map (see https://stackoverflow.com/questions/33185037/how-to-set-default-colormap-in-matplotlib). Unless you do that you would have to specify it as you suggest below.
Instead of binary, you might want to use gray, e.g:
show_image(img,cmap='gray')