Summary:
Updates the remaining references to Exponent to Expo, similar in spirit to b698b4ad62. Left the filename of the blog post alone so that permalinks keep working.
Closes https://github.com/facebook/react-native/pull/12776
Differential Revision: D4676911
Pulled By: hramos
fbshipit-source-id: 5b8b8413f95c984df4157e4e1634dd35a78afa29