doc: polyunfill: point to https://github.com/NixOS/nixpkgs/pull/314791

hosts/common/polyunfill.nix

@@ -53,7 +53,8 @@ in
       ])) services;
     in lib.mapAttrs (_serviceName: service: service // {
       # replace references with the old pam_unix, which calls into /run/wrappers/bin/unix_chkpwd,
-      # with a pam_unix that calls until unix_chkpwd via the nix store.
+      # with a pam_unix that calls into unix_chkpwd via the nix store.
+      # TODO: use `security.pam.package` instead once <https://github.com/NixOS/nixpkgs/pull/314791> lands.
       text = lib.replaceStrings [" pam_unix.so" ] [ " ${suidlessPam}/lib/security/pam_unix.so" ] service.text;
     }) filtered;
   };