cmd/gorename: fix test failure formatting.

Change-Id: Idbfb356225f4650446bffa80a43cec4679bcd507
Reviewed-on: https://go-review.googlesource.com/75976
Reviewed-by: Alan Donovan <adonovan@google.com>
diff --git a/cmd/gorename/gorename_test.go b/cmd/gorename/gorename_test.go
index 20dbd9e..b778cb6 100644
--- a/cmd/gorename/gorename_test.go
+++ b/cmd/gorename/gorename_test.go
@@ -370,7 +370,7 @@
 			file := filepath.Join(pkgDir, strconv.Itoa(i)+".go")
 			// read file contents and compare to val
 			if contents, err := ioutil.ReadFile(file); err != nil {
-				t.Fatal("File missing: %s", err)
+				t.Fatalf("File missing: %s", err)
 			} else if string(contents) != val {
 				results = append(results, strings.TrimPrefix(dir, file))
 			}