docs(compat): fix broken auth link

[hendrickson-tyler]

Checklist

• Issue number for this PR: Trivial fix, can create an issue if required
• Docs included?: Yes
• Test units included?: None required
• In a clean directory, yarn install, yarn test run successfully? Yes

Description

Currently the "Getting started with Firebase Authentication" link in the compat docs returns a 404 because of a typo.

Code sample

Fixed a typo from

[Getting started with Firebase Authentication](acompat/uth/getting-started.md)

to

[Getting started with Firebase Authentication](compat/auth/getting-started.md)