zls install instruction for neovim should no longer recommend nvim-lua/completion-nvim

[bauripalash]

The Neovim plugin https://github.com/nvim-lua/completion-nvim has been archived and is no longer maintained. The Author and the official docs of neovim lsp-config^1 recommends https://github.com/hrsh7th/nvim-cmp/. Please update the site.