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/asteroids.rs

@@ -1,9 +1,10 @@
+//! This is the module containing all the rock-related things.
+//! Not... not the whole game.
+
 use bevy_rapier2d::prelude::*;
 use rand::{Rng, SeedableRng};
 use std::time::Duration;
 
-/// This is the module containing all the rock-related things
-/// not... not the whole game.
 use bevy::prelude::*;
 
 use crate::{GameAssets, Rotation, WorldSize, physics::Position, physics::Velocity};

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;