Usage example has wrong CLI args

See https://gitlab.com/brainelectronics/lightweight-versioned-gitlab-pages/-/blob/85fc8c4b5fcf946f71b1dcec777403c0ae8a34f9/README.md#L88 and https://gitlab.com/brainelectronics/lightweight-versioned-gitlab-pages/-/blob/85fc8c4b5fcf946f71b1dcec777403c0ae8a34f9/README.md#L104

Both use --private_token but the correct argument is --private-token