README: Updated dead link for nix channel manual (NixOS#379247)

* README: Updated dead link for nix channel

The old link from the manual lead to a dead page.

The new link leads to the nix-channel command on the manual.
However it would also be reasonable for it to lead to the wiki[1] entry on nix channels, or on the channels section of the FAQ[2], however the ladder option already leads to the nix-channel command manual entry "for more information".

[1]: https://nixos.wiki/wiki/Nix_channels
[2]: https://nix.dev/concepts/faq#channel-branches

* README: link to stable branch of the manual instead of a specific version

Author: PedroManse

README.md

@@ -59,7 +59,7 @@ system, [Hydra](https://hydra.nixos.org/).
 Artifacts successfully built with Hydra are published to cache at
 https://cache.nixos.org/. When successful build and test criteria are
 met, the Nixpkgs expressions are distributed via [Nix
-channels](https://nixos.org/manual/nix/stable/package-management/channels.html).
+channels](https://nix.dev/manual/nix/stable/command-ref/nix-channel.html).
 
 # Contributing