Fix spelling of skscene in README

[paulcyi]

@lpuentes19
@armadsen

Luis:

Here is the finished project for Module Challenge: iOS Development Tools - Sprites. All stretch goals have been completed.

Andrew:

Per your recommendation, here is the pull request that fixes the spelling error for skscene in the view controller class code:

previous - > skview.presentScene(skScene)

revised -> skview.presentScene(skscene)

var skscene: CustomScene? = nil

override func viewDidAppear(_ animated: Bool) {
    super.viewDidAppear(animated)
    skscene = CustomScene(size: view.bounds.size)
    skview.presentScene(skscene)
}