React Native DataStore docs missing dependency installation

[david-mcafee]

Describe the content issue:
A clear and concise description of what the content issue is. Is there content missing or is there incorrect content?
After following the DataStore Getting Started section, the created app will not compile. The app is missing amazon-cognito-identity-js, resulting in the following error: Possible Unhandled Promise Rejection (id: 0): Error: Could not find a native getRandomBase64 implementation. Validate that amazon-cognito-identity-js is linked.

URL page where content issue is:
https://docs.amplify.aws/lib/datastore/getting-started/q/platform/react-native/

[david-mcafee]

PR has been merged.