doc: don't use sha256 and non-sri hashes in user docs (#522716)

nixos/doc/manual/release-notes/rl-2605.section.md

@@ -23,11 +23,11 @@
     # Pinned Nixpkgs archive
     #
     # Use `curl -I https://channels.nixos.org/nixos-26.05` to get the
-    # latest commit of the stable channel and `nix-prefetch-url --unpack`
-    # to compute its sha256 hash.
+    # latest commit of the stable channel and `nix --extra-experimental-features nix-command store prefetch-file --unpack`
+    # to compute its hash.
     nixpkgs = builtins.fetchTarball {
       url = "https://github.com/NixOS/nixpkgs/archive/c217913993d6.tar.gz";
-      sha256 = "026mprs324330pfazlgbw987qmsa8ligglarvqbcxzig2kgw0lqg";
+      hash = "sha256-D1PA3xQv/s4W3lnR9yJFSld8UOLr0a/cBWMQMXS+1Qg=";
     };
   in
   import "${nixpkgs}/nixos" {