Fix module-level docstring in asteroids & config

I made a comment to help me remember the purpose, but if I do it this
way `cargo doc` will render it out to the actual documentation.

src/config.rs

@@ -1,7 +1,5 @@
-/*
- Global constants used all over the program. Rather than leaving them scattered
-where ever they happen to be needed, I'm concentrating them here.
-*/
+//! Global constants used all over the program. Rather than leaving them scattered
+//! where ever they happen to be needed, I'm concentrating them here.
 
 use bevy::color::Color;