gob: trivial print fix

R=golang-dev, gri
CC=golang-dev
https://golang.org/cl/5707062

src/pkg/encoding/gob/encoder_test.go

@@ -709,7 +709,7 @@ func TestGobPtrSlices(t *testing.T) {
 		t.Fatal("decode:", err)
 	}
 	if !reflect.DeepEqual(in, out) {
-		t.Fatal("got %v; wanted %v", out, in)
+		t.Fatalf("got %v; wanted %v", out, in)
 	}
 }