change in in_region cost example

Chapter07/in_region_cost_example.py

@@ -17,7 +17,7 @@ layers = [slgbuilder.GraphObject(0*I), slgbuilder.GraphObject(0*I)] # no on-surf
 helper = slgbuilder.MaxflowBuilder()
 helper.add_objects(layers)
 
-# Addin regional costs, 
+# Adding regional costs, 
 # the region in the middle is bright compared to two darker regions.
 helper.add_layered_region_cost(layers[0], I, 255-I)
 helper.add_layered_region_cost(layers[1], 255-I, I)