Move nbody description to a normal comment

For consistency with other benchmarks

Author: Kobzol

collector/runtime-benchmarks/nbody/src/main.rs

@@ -1,12 +1,11 @@
-//! Calculates the N-body simulation.
-//! Code taken from https://github.com/prestontw/rust-nbody
-
 use benchlib::benchmark::run_benchmark_group;
 
 mod nbody;
 
 fn main() {
     run_benchmark_group(|group| {
+        // Calculates the N-body simulation.
+        // Code taken from https://github.com/prestontw/rust-nbody
         group.register_benchmark("nbody_5k", || {
             let mut nbody = nbody::init(5000);
             || {