docs: unify Node version

code examples and docs had different Node versions so I fixed it as it is a little bit confusing If I missed or misunderstood anything and this is actually correct, I apologize deeply. Please correct me if I am wrong. 🙏 Here are the two changed sections: - Look at almost the end of https://nixos-and-flakes-book-git-fork-truebe-f1314b-ryan4yins-projects.vercel.app/development/intro#creating-a-development-environment - At the start of this section after the code block: https://nixos-and-flakes-book-git-fork-truebe-f1314b-ryan4yins-projects.vercel.app/development/intro#creating-a-development-environment-with-pkgs-runcommand