nixos/doc/rl-25.05: fix typo (#399724)

- `nixos-rebuild-ng`, a full rewrite of `nixos-rebuild` in Python, is available for testing. You can enable it by setting [system.rebuild.enableNg](options.html#opt-system.rebuild.enableNg) in your configuration (this will replace the old `nixos-rebuild`), or by adding `nixos-rebuild-ng` to your `environment.systemPackages` (in this case, it will live side-by-side with `nixos-rebuild` as `nixos-rebuild-ng`). It is expected that the next major version of NixOS (25.11) will enable `system.rebuild.enableNg` by default.

- The `nixos-generate-config` command now supports a optional `--flake` option, which will generate a flake.nix file alongside the `configuration.nix` and `hardware-configuration.nix`, providing an easy instroduction into flake-based system configurations.

- The `nixos-generate-config` command now supports a optional `--flake` option, which will generate a flake.nix file alongside the `configuration.nix` and `hardware-configuration.nix`, providing an easy introduction into flake-based system configurations.

- A `nixos-rebuild build-image` sub-command has been added.

  It allows users to build platform-specific (disk) images from their NixOS configurations. `nixos-rebuild build-image` works similar to the popular [nix-community/nixos-generators](https://github.com/nix-community/nixos-generators) project. See new [section on image building in the NixOS manual](https://nixos.org/manual/nixos/unstable/#sec-image-nixos-rebuild-build-image). It is also available for `nixos-rebuild-ng`.