docs(auth): missing AngularFireAuthModule import

[luizcieslak]

Checklist

• Issue number for this PR: Readme sample code throws error: Runtime Error Uncaught (in promise): Error: No provider for AngularFireAuth!  #956
• Docs included?: yes, this is a docs change
• Test units included?: no
• e2e tests included?: no
• In a clean directory, npm install, npm run build, and npm test run successfully? yes

Description

Missing AngularFireAuthModule import at @NgModule which causes an uncaught Runtime Error when using Ionic3.

Code sample

n/a