doc(livebook): Fix small spelling mistake (#342373)

      type = with lib.types; nullOr lib.types.path;

      default = null;

      description = ''

        Additional dnvironment file as defined in {manpage}`systemd.exec(5)`.

        Additional environment file as defined in {manpage}`systemd.exec(5)`.

        Secrets like {env}`LIVEBOOK_PASSWORD` (which is used to specify the

        password needed to access the livebook site) or {env}`LIVEBOOK_COOKIE`