docs(useAuth): fix typo on usage section (vitest-dev#668)

Author: excessivecoding

packages/firebase/useAuth/index.md

@@ -14,7 +14,7 @@ can easily react to changes in the users' authentication status.
 import firebase from 'firebase'
 import { useAuth } from '@vueuse/firebase/useAuth'
 
-const { GoogleAuthProvider } = auth
+const { GoogleAuthProvider } = firebase.auth
 
 const { isAuthenticated, user } = useAuth(firebase.auth)