Fix tiny typo in demo example

docs/examples/core/example-demo.py

@@ -31,7 +31,7 b" print 'This is an automatic block.'"
 print 'It is executed without asking for confirmation, but printed.'
 z = x+y
 
-print 'z=',x
+print 'z=',z
 
 # <demo> --- stop ---
 # This is just another normal block.