fix: use absolute URLs for assets

this will work in the PDF when exported locally whereas before it
wasn't. they're in a reliable place, so it should be fine to do.

src/03-spit-fire.md

@@ -136,7 +136,9 @@ With that refactor done, let's display a sprite for our fireball and call it a c
 
 Download the fireball sprite below and put it in `mygame/sprites/fireball.png`:
 
-![fireball sprite](./code/chapter_03/06_sprite/sprites/fireball.png)
+![fireball sprite](https://book.dragonriders.community/code/chapter_03/06_sprite/sprites/fireball.png)
+
+[Download sprite](https://book.dragonriders.community/code/chapter_03/06_sprite/sprites/fireball.png)
 
 I just made that! It's not half bad, right?
 

src/08-sound.md

@@ -12,10 +12,10 @@ We'll also add a music track that plays in the background.
 
 Download the following files and put them in the `mygame/sounds/` folder:
 
-- [fireball.wav](./code/chapter_08/01_sound/sounds/fireball.wav)
-- [target.wav](./code/chapter_08/01_sound/sounds/target.wav)
-- [game-over.wav](./code/chapter_08/01_sound/sounds/game-over.wav)
-- [flight.ogg](./code/chapter_08/01_sound/sounds/flight.ogg)
+- [fireball.wav](https://book.dragonriders.community/code/chapter_08/01_sound/sounds/fireball.wav)
+- [target.wav](https://book.dragonriders.community/code/chapter_08/01_sound/sounds/target.wav)
+- [game-over.wav](https://book.dragonriders.community/code/chapter_08/01_sound/sounds/game-over.wav)
+- [flight.ogg](https://book.dragonriders.community/code/chapter_08/01_sound/sounds/flight.ogg)
 
 A note on audio file formats: in DragonRuby, sound effects are generally WAV (`.wav`) files. WAV files are uncompressed, meaning they can be quite large in size but are high-quality. Because sound effects are so short, the file size is less of a concern. Music files are OGG (`.ogg`), an open-source format for audio that works across all of the platforms your game can run on. OGG files are compressed, thus smaller in size than WAV files, making them good for music tracks.