readme: update reference to number of packages

We updated the top of the readme to reference 60,000, but we forgot to
update this reference. This changes it so it won't become invalid for
a bit.

README.md

@@ -60,7 +60,7 @@ channels](https://nixos.org/nix/manual/#sec-channels).
 Nixpkgs is among the most active projects on GitHub. While thousands
 of open issues and pull requests might seem a lot at first, it helps
 consider it in the context of the scope of the project. Nixpkgs
-describes how to build over 40,000 pieces of software and implements a
+describes how to build tens of thousands of pieces of software and implements a
 Linux distribution. The [GitHub Insights](https://github.com/NixOS/nixpkgs/pulse)
 page gives a sense of the project activity.