Update SLIC in example to most recent interface

SLIC has been updated a few times since this example was created,
adding support for grayscale images (so converting to RGB is no longer
necessary) and changing the keyword "ratio" to "compactness". This
commit brings the example up to date.

doc/examples/plot_join_segmentations.py

@@ -20,7 +20,6 @@ from skimage.morphology import watershed
 from skimage.color import label2rgb
 from skimage import data
 
-
 coins = data.coins()
 
 # make segmentation using edge-detection and watershed
@@ -34,11 +33,8 @@ ws = watershed(edges, markers)
 seg1 = nd.label(ws == foreground)[0]
 
 # make segmentation using SLIC superpixels
-
-# make the RGB equivalent of `coins`
-coins_colour = np.tile(coins[..., np.newaxis], (1, 1, 3))
-seg2 = slic(coins_colour, n_segments=30, max_iter=160, sigma=1, ratio=9,
-            convert2lab=False)
+seg2 = slic(coins, n_segments=117, max_iter=160, sigma=1, compactness=0.75,
+            multichannel=False)
 
 # combine the two
 segj = join_segmentations(seg1, seg2)