README.md: NixCon https -> http (#217)

The certificate looks to have expired or something, so the https
redirect for NixCon's site is broken. This should work as a stopgap
until <nixcon/nixcon.org#2> is fixed.

README.md

@@ -270,5 +270,5 @@
 * [#nixos on Libera.Chat](https://web.libera.chat/?nick=Guest?#nixos)
 * [Discord - Nix/Nixos (Unofficial)](https://discord.gg/BMUCQx6)
 * [Discourse](https://discourse.nixos.org/) - The best place to get help and discuss Nix-related topics.
-* [NixCon](https://nixcon.org/) - The annual community conference for contributors and users of Nix and NixOS.
+* [NixCon](http://nixcon.org/) - The annual community conference for contributors and users of Nix and NixOS.
 * [Wiki (Unofficial)](https://nixos.wiki)