Add missing ctx variable in the func main() in the "introduction to acceptance tests" chapter

[UponTheSky]

While I was reading the chapter "Introduction to acceptance tests" with actual coding, I found the following missing part:

According to example code of Graceful shutdown decorator package, I would like to suggest the same code example in the book as well:

func main() {
  var (
    ctx        = context.Background()
    httpServer = &http.Server{Addr: ":8080", Handler: http.HandlerFunc(acceptancetests.SlowHandler)}
    server     = gracefulshutdown.NewServer(httpServer)
  )
  // code omitted
}