minor fixes to platform docs
minor fixes to platform docs

Summary:
platform docs which was copied from RN has some places where it didn't specify the vr changes
Closes https://github.com/facebook/react-vr/pull/123

Reviewed By: amberroy

Differential Revision: D4935547

Pulled By: amberroy

fbshipit-source-id: 4b39ef5

ichengde/react-vr,ichengde/react-vr