docs: cssnano options links to casino site

https://github.com/webpack/css-minimizer-webpack-plugin/blob/86ee07da1ba7d856a1942dd1a283bab0df29b21b/README.md?plain=1#L319

Corrected link https://cssnano.github.io/cssnano/docs/what-are-optimisations/#what-are-optimisations

leftover from https://github.com/webpack/css-minimizer-webpack-plugin/pull/260