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/string.rs

@@ -1,10 +1,9 @@
+//! This module implements Nix language strings and their different
+//! backing implementations.
 use smol_str::SmolStr;
 use std::hash::Hash;
 use std::{borrow::Cow, fmt::Display};
 
-/// This module implements Nix language strings and their different
-/// backing implementations.
-
 #[derive(Clone, Debug)]
 enum StringRepr {
     Smol(SmolStr),