os/exec: fix nit found by vet

Change-Id: I8085ed43d63215237a4871cc1e44257132a7f5de
Reviewed-on: https://go-review.googlesource.com/27130Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/os/exec/exec_test.go

@@ -938,7 +938,7 @@ func TestContextCancel(t *testing.T) {
 	}
 
 	if err := w.Close(); err != nil {
-		t.Error("error closing write end of pipe: %v", err)
+		t.Errorf("error closing write end of pipe: %v", err)
 	}
 	<-readDone