path: fix printf glitch in test

R=rh, gri
CC=golang-dev
https://golang.org/cl/3514041

src/pkg/path/path_test.go

@@ -257,7 +257,7 @@ func TestWalk(t *testing.T) {
 	errors := make(chan os.Error, 64)
 	Walk(tree.name, v, errors)
 	if err, ok := <-errors; ok {
-		t.Error("no error expected, found: s", err)
+		t.Errorf("no error expected, found: %s", err)
 	}
 	checkMarks(t)