transport: fixed link in the examples_test.go

Change-Id: Idaa1422f0cf068dbfc765059e164d72f519587aa
Reviewed-on: https://code-review.googlesource.com/c/google-api-go-client/+/42332
Reviewed-by: Jean de Klerk <deklerk@google.com>
diff --git a/transport/examples_test.go b/transport/examples_test.go
index 11b69eb..fe30ccf 100644
--- a/transport/examples_test.go
+++ b/transport/examples_test.go
@@ -40,7 +40,7 @@
 func Example_withCredentialsFile() {
 	ctx := context.Background()
 
-	// Download service account creds per https://cloud.google.com/docs/authentication/end-user.
+	// Download service account creds per https://cloud.google.com/docs/authentication/production.
 	//
 	// Note: Given the same set of options, transport.NewHTTPClient and
 	// transport.DialGRPC use the same credentials.