examples: replace all app.Run(iris.Addr(...)) with app.Listen just for the shake of simplicity, both are doing the same exact thing as it's described on the http listening first example.

Former-commit-id: d20afb2e899aee658a8e0ed1693357798df93462

_examples/tutorial/online-visitors/main.go

@@ -47,7 +47,7 @@ func main() {
 	// Each page has its own online-visitors counter.
 	app.Get("/", h)
 	app.Get("/other", h2)
-	app.Run(iris.Addr(":8080"))
+	app.Listen(":8080")
 }
 
 type page struct {