nixos/image: fix documentation build

literalExpression triggers the following error when building the
manual:

Cacheable portion of option doc build failed.
Usually this means that an option attribute that ends up in documentation (eg `default` or `description`) depends on the restricted module arguments `config` or `pkgs`.

nixos/modules/image/repart.nix

@@ -90,9 +90,7 @@ in
 
     package = lib.mkPackageOption pkgs "systemd-repart" {
       default = "systemd";
-      example = lib.literalExpression ''
-        pkgs.systemdMinimal.override { withCryptsetup = true; }
-      '';
+      example = "pkgs.systemdMinimal.override { withCryptsetup = true; }";
     };
 
     partitions = lib.mkOption {