Update auth.md change user$ = user(auth); to user$ = user(this.auth);

[arolleaguekeng]

Checklist

• Issue number for this PR: #3640
• Docs included?: no
• Test units included?: no
• In a clean directory, yarn install, yarn test run successfully? yes

Description

This PR fixes an issue in the AngularFire documentation where the auth instance was incorrectly referenced as user(auth), instead of using the injected instance (this.auth).

Description

Code sample