buildDhallDirectoryPackage: Stringify src for documentationRoot (#162401)

The `src` is supposed to be a path, but when `documentationRoot` is provided
with a path, it errors with:

generators.mkValueStringDefault: this value is not supported: "<nix-store-path>"

Making it a string fixes this error.

pkgs/development/interpreters/dhall/build-dhall-directory-package.nix

···
           code = "${src}/${file}";
 
         }
-      // lib.optionalAttrs document { documentationRoot = src; }
+      // lib.optionalAttrs document { documentationRoot = "${src}"; }
       )
   )
-