docs(tvix/eval): Use correct syntax for module doc comments

Change-Id: I35741856f34b86a538f226a8eaf8806edede60ec
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6207
Reviewed-by: grfn <grfn@gws.fyi>
Tested-by: BuildkiteCI

tvix/eval/src/value/list.rs

@@ -1,4 +1,4 @@
-/// This module implements Nix lists.
+//! This module implements Nix lists.
 use std::fmt::Display;
 
 use super::Value;