gitiles: fix vet error

Change-Id: I865856998107476434c624f7c3686c9d794f4f73

gitiles/prod_test.go

diff --git a/gitiles/prod_test.go b/gitiles/prod_test.go
index 43ccbef..efae4af 100644
--- a/gitiles/prod_test.go
+++ b/gitiles/prod_test.go
@@ -56,7 +56,7 @@
 	}
 
 	if !names["mux.go"] {
-		t.Fatal("did not find 'mux.go', got %v", names)
+		t.Fatalf("did not find 'mux.go', got %v", names)
 	}
 }