IT keeps asking me to change the var to a constant and when i tried that it didnt run

class ViewController: UIViewController {

    @IBOutlet weak var funFactLabel: UILabel!

    let factBook = FactBook()


    override func viewDidLoad() {
        super.viewDidLoad()
            funFactLabel.text = factBook.randomFact()
    }

    override func didReceiveMemoryWarning() {
        super.didReceiveMemoryWarning()
        // Dispose of any resources that can be recreated.
    }


    @IBAction func showFunFact() {
        funFactLabel.text = factBook.randomFact()
    }
}

struct FactBook {
    let factsArray = [
        "Ants stretch when they wake up in the morning.",
        "Ostriches can run faster than horses.",
        "Olympic gold medals are actually made mostly of silver.",
        "You are born with 300 bones; by the time you are an adult you will have 206.",
        "It takes about 8 minutes for light from the Sun to reach Earth.",
        "Some bamboo plants can grow almost a meter in just one day.",
        "The state of Florida is bigger than England.",
        "Some penguins can leap 2-3 meters out of the water.",
        "On average, it takes 66 days to form a new habit.",
        "Mammoths still walked the earth when the Great Pyramid was being built."]
    func randomFact() -> String {
        var unsignedArrayCount = UInt32(factsArray.count)
        var unsignRandomNumber = arc4random_uniform(unsignedArrayCount)
        var randomNumber = Int(unsignRandomNumber)

        return factsArray[randomNumber]
    }