Summary:
Small fix. Adds a proptype for the injectJavaScript function, added in da9a712a9e17942dcd05b8d955f0764c2026a4ad.
Closes https://github.com/facebook/react-native/pull/12665
Differential Revision: D4645803
Pulled By: hramos
fbshipit-source-id: 863e5bf82b853914ac08f9f5b2c141d35882afc7