Fix _text_ in Koran translation.

`_text_` doesn't work when it's in the middle of a word: e.g. Abc_def_

In Korean translation, there's so many cases due to *postposition* (-이/가, -을/를, -은/는..) in Korean language system.

Note `*text*` works: e.g. Abc*def*.

Ideas:
- use `*text*` instead
- fix MD parser to support `_text_`

Btw, in Korean, we don't use _italic_ to emphasize in general. Instead, we use **bold** or 'quote'. I'd like to avoid _italic_ in Korean translation.