Merge pull request #144240 from Mic92/readme

README: sync package count with https://search.nixos.org/packages

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README.md
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</p>
[Nixpkgs](https://github.com/nixos/nixpkgs) is a collection of over
-
60,000 software packages that can be installed with the
+
80,000 software packages that can be installed with the
[Nix](https://nixos.org/nix/) package manager. It also implements
[NixOS](https://nixos.org/nixos/), a purely-functional Linux distribution.