Finish refactoring UseRegistry

go/src/demos/cmd/simple/main.go

@@ -26,7 +26,7 @@ func main() {
 		Extra("style", registry.M{"color": "#a77"})
 
 	f := flow.New()
-	f.SetRegistry(r)
+	f.UseRegistry(r)
 
 	op := f.Op("Join",
 		f.Op("Split", "hello world", " "),