consistent use of finall class

Summary:
You are already using `final class` as the default swift declaration as I guess most people agree this is best practice for classes you are not planning to inherit from :) So I just made the example code more consistent.

I will not add much more reasoning to why, other than open source projects like this is a _big_ inspirations and starting ground to play with swift for new develoeprs joining the iOS community. Nuding them to ask the right questions (composition vs inheritance) early could only be a good thing (?)

- [X] All tests pass. Demo project builds and runs.
- [X] No new tests. No new classes.
- [X] No breaking changes.
- [X] I have reviewed the [contributing guide](https://github.com/Instagram/IGListKit/blob/master/.github/CONTRIBUTING.md)
Closes https://github.com/Instagram/IGListKit/pull/539

Differential Revision: D4696591

Pulled By: rnystrom

fbshipit-source-id: 73539b907452ec0cb7a8e842a95cb4fd27138f2f

Examples/Examples-iOS/IGListKitExamples/SectionControllers/GridSectionController.swift

@@ -15,7 +15,7 @@
 import UIKit
 import IGListKit
 
-class GridItem: NSObject {
+final class GridItem: NSObject {
 
     let color: UIColor
     let itemCount: Int