doc/progs: fix windows/amd64

R=rsc
CC=alex.brainman, golang-dev, vcc.163
https://golang.org/cl/4965050

doc/progs/file_windows.go

@@ -15,7 +15,7 @@ type File struct {
 }
 
 func newFile(fd syscall.Handle, name string) *File {
-	if fd < 0 {
+	if fd == ^syscall.Handle(0) {
 		return nil
 	}
 	return &File{fd, name}

doc/progs/run

@@ -15,7 +15,7 @@ fi
 rm -f *.$O
 
 if [ "$GOOS" = "windows" ];then
-	$GC -o file.8 file_windows.go
+	$GC -o file.$O file_windows.go
 else
 	$GC file.go
 fi