fix link to PyBoy.pdf (#248)

README.md

@@ -184,7 +184,7 @@ You can find a (way cooler) web version of this list [here](https://gbdev.github
 ## Emulator Development
 
 - [Reverse Engineering fine details of Game Boy hardware](https://www.youtube.com/watch?v=GBYwjch6oEE) - 43 minutes talk by Gekkio given at Disobey 2018 ([errata](https://gekkio.fi/blog/2018-02-05-errata-for-reverse-engineering-fine-details-of-game-boy-hardware.html)).
-- [Emulation of Nintendo Game Boy](https://github.com/Baekalfen/PyBoy/blob/master/PyBoy.pdf) - Overview of the Game Boy hardware with the perspective of building an emulator.
+- [Emulation of Nintendo Game Boy](https://github.com/Baekalfen/PyBoy/blob/master/extras/PyBoy.pdf) - Overview of the Game Boy hardware with the perspective of building an emulator.
 - [DMG-01](https://rylev.github.io/DMG-01/public/book/) - An educational Gameboy Emulator in Rust and a companion book explaining its development. *[Oh Boy! Creating a Game Boy Emulator in Rust](https://media.ccc.de/v/rustfest-rome-3-gameboy-emulator)- is a talk given at Rust Fest 18 about this.
 - [Building a Game Boy emulator in JavaScript](http://imrannazar.com/gameboy-Emulation-in-JavaScript) - Step by step tutorial.
 - [Writing a Game Boy emulator, Cinoop](https://cturt.github.io/cinoop.html)