Merge pull request #194159 from foo-dogsquared/update-systemd-extraconfig-module-doc

nixos/systemd: update extraConfig description

nixos/modules/system/boot/systemd.nix

···
       type = types.lines;
       example = "DefaultLimitCORE=infinity";
       description = lib.mdDoc ''
-        Extra config options for systemd. See man systemd-system.conf for
-        available options.
+        Extra config options for systemd. See systemd-system.conf(5) man page
+        for available options.
       '';
     };