cmd/pack: fix format string error in log message

Fixes #7693.

pack.go:347: possible formatting directive in Fatal call

LGTM=iant
R=golang-codereviews, iant
CC=golang-codereviews
https://golang.org/cl/83310045

src/cmd/pack/pack.go

@@ -344,7 +344,7 @@ func (ar *Archive) addFile(fd FileLike) {
 		log.Fatal("writing file: ", err)
 	}
 	if n64 != info.Size() {
-		log.Fatal("writing file: wrote %d bytes; file is size %d", n64, info.Size())
+		log.Fatalf("writing file: wrote %d bytes; file is size %d", n64, info.Size())
 	}
 	ar.endFile()
 }