Fix guides docs to es2015 classes and remove flowtype from Animation example

Summary:
1. Animation guide page is the only place where Flowtype is used, it would be better to remove it to prevent some confusion.

2. ES2015 classes in guidelines docs pages and fixed some typos

**Test plan (required)**

Should i write any tests for this?
Closes https://github.com/facebook/react-native/pull/8339

Differential Revision: D3474192

Pulled By: bestander

fbshipit-source-id: 5531d1e399eaed0952732ac2e0bd1effc72d00a8