doc: rust: improve clarity of example snippet

Make it more clear that the return type is a string.

Co-authored-by: figsoda <figsoda@pm.me>

doc/languages-frameworks/rust.section.md

@@ -142,7 +142,7 @@ rustPlatform.buildRustPackage rec {
   version = "1.0.0";
 
   cargoLock = let
-    fixupLockFile = path: /* ... */;
+    fixupLockFile = path: f (builtins.readFile path);
   in {
     lockFileContents = fixupLockFile ./Cargo.lock;
   };