Fixed warning in doc example

doc/tutorial/examples.txt

@@ -489,7 +489,7 @@ The preceding elements are featured in this more realistic example.  It will be
   train = theano.function(
             inputs=[x,y],
             outputs=[prediction, xent],
-            updates={w: w - 0.1 * gw, b: b - 0.1 * gb})
+            updates=((w, w - 0.1 * gw), (b, b - 0.1 * gb)))
   predict = theano.function(inputs=[x], outputs=prediction)
 
   # Train