Use type checking instead of casting in README
Use type checking instead of casting in README

Summary:
Just a little improvement to a code example.

- [x] All tests pass. Demo project builds and runs.
- [x] I added tests, an experiment, or detailed why my change isn't tested.
- [x] I have reviewed the [contributing guide](https://github.com/Instagram/IGListKit/blob/master/CONTRIBUTING.md)
Closes https://github.com/Instagram/IGListKit/pull/45

Reviewed By: jessesquires

Differential Revision: D4002830

Pulled By: rnystrom

fbshipit-source-id: b6fb8f87752f0886333c4445107caac8219027e9

Sherlouk/IGListKit,dshahidehpour/IGListKit,zhubofei/IGListKit,zhubofei/IGListKit,Sherlouk/IGListKit,dshahidehpour/IGListKit,Instagram/IGListKit,charles0126dev/IGListKit,zhubofei/IGListKit,Instagram/IGListKit,Sherlouk/IGListKit,dshahidehpour/IGListKit,zhubofei/IGListKit,Instagram/IGListKit,zhubofei/IGListKit,Instagram/IGListKit,charles0126dev/IGListKit,charles0126dev/IGListKit,Instagram/IGListKit