Update homebrew installation

This package was included in the official [homebrew repository](https://formulae.brew.sh/formula/cowsay). I think it would be nice to update the respective sections in [README](https://github.com/cowsay-org/cowsay?tab=readme-ov-file#installation-using-homebrew) and on the [webpage](https://github.com/cowsay-org/cowsay-www).