README: update matrix link (#393815)

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README.md
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# Community
* [Discourse Forum](https://discourse.nixos.org/)
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* [Matrix Chat](https://matrix.to/#/#community:nixos.org)
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* [Matrix Chat](https://matrix.to/#/#space:nixos.org)
* [NixOS Weekly](https://weekly.nixos.org/)
* [Official wiki](https://wiki.nixos.org/)
* [Community-maintained list of ways to get in touch](https://wiki.nixos.org/wiki/Get_In_Touch#Chat) (Discord, Telegram, IRC, etc.)