Fix-up some tkinter demos.
diff --git a/Demo/tkinter/guido/brownian2.py b/Demo/tkinter/guido/brownian2.py
index dc1d43a..3adcd68 100644
--- a/Demo/tkinter/guido/brownian2.py
+++ b/Demo/tkinter/guido/brownian2.py
@@ -32,7 +32,7 @@
             yield None
 
 def move(particle): # move the particle at random time
-    particle.next()
+    next(particle)
     dt = random.expovariate(LAMBDA)
     root.after(int(dt*1000), move, particle)