Update Auth's getting-starder.md

[laulaaa]

In firebase @4.12.0"
The function firebaseAuth.authState returns an Observable<firebase.User>

In firebase @4.13.1"
The function firebaseAuth.authState returns an Observable< User>
User comes from firebase/auth-types

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[googlebot]

Thanks for your pull request. It looks like this may be your first contribution to a Google open source project (if not, look below for help). Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

📝 Please visit https://cla.developers.google.com/ to sign.

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What to do if you already signed the CLA

Individual signers

• It's possible we don't have your GitHub username or you're using a different email address on your commit. Check your existing CLA data and verify that your email is set on your git commits.

Corporate signers

• Your company has a Point of Contact who decides which employees are authorized to participate. Ask your POC to be added to the group of authorized contributors. If you don't know who your Point of Contact is, direct the Google project maintainer to go/cla#troubleshoot (Public version).
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[jamesdaniels]

@laulaaa have a chance to look at the CLA yet?

[jamesdaniels]

Thanks for the PR, I've changed the API interface for AngularFireAuth in #1642 though; closing since this will be outdated once we merge that. Feel free to continue doc changes and reopen when it makes sense.

Thanks for contributing 👍