[docs] Update link in README.md for FirebaseAuth SampleSwift. (#13746)

Author: lamtrinhdev

FirebaseAuth/README.md

@@ -13,5 +13,5 @@ Example/Auth contains a set of samples and tests that integrate with
 FirebaseAuth.
 
 The unit tests run without any additional configuration along with the rest of
-Firebase. See [Tests/Sample/README.md](Tests/Sample/README.md) for
+Firebase. See [Tests/SampleSwift/README.md](Tests/SampleSwift/README.md) for
 information about setting up, running, and testing the samples.