Fix ftw.book icons: interchange "PDF" and "Read" icons.

The "PDF" and "Read" document_action icons were wrong, I've swapped them.

New:
![bildschirmfoto 2014-02-24 um 09 40 10](https://f.cloud.github.com/assets/7469/2243582/760c0760-9d2f-11e3-939b-998073d8f843.png)