wrong code demo in model demo.

[zhoupro]

https://beego.me/docs/quickstart/model.md

` // Package main provides ... package main

import ( "fmt" "path/filepath" )

func main() { ext := filepath.Ext("http://www.google.com/search.go") fmt.Println(ext) }

` The output will be .go rather go. So the demo will always be true.