Update compat.md

[jeremy-kunzhou]

trafficstars

Checklist

• Issue number for this PR: #nnn (required) NA
• Docs included?: (yes/no; required for all API/functional changes) yes
• Test units included?: (yes/no; required) not required. doc change only
• In a clean directory, yarn install, yarn test run successfully? (yes/no; required) doc change only

Description

Fixed the link for Getting started with Firebase Authentication in compat.md

Code sample

NA

[jeremy-kunzhou]

@nohe427 Thanks.