Summary:
Update Integration with Existing Apps
Pod File missing link to podspecs that are not in the master CocoaPods spec repo
Be able to trust documentation when I come back to the page later.
[DOCS][ENHANCEMENT][docs/IntegrationWithExistingApps.md] Add private podspec path to iOS 'Integration with Existing Apps' example.
Closes https://github.com/facebook/react-native/pull/16461
Differential Revision: D6099907
Pulled By: hramos
fbshipit-source-id: f5ca94465355172e9d1df39469b66aa3b6a8624c