fix the garbled characters with multibyte characters in preamble

I will use mizuho to convert the text which has multibyte characters.
Unfortunately, if I write the text in preamble, mizuho make the garbled characters.

Here is the text which I used.

```
Guide
=====
foo <foo@example.com>

Hello, world.
こんにちは。

Summary
-------

Hello, world.
こんにちは。
```


Thanks.