minimal-bootstrap.ln-boot: fix typo in usage

pkgs/os-specific/linux/minimal-bootstrap/ln-boot/default.nix

···
 }:
 let
   pname = "ln-boot";
-  version = "unstable-2023-05-01";
+  version = "unstable-2023-05-22";
 
   src = ./ln.c;
 in

pkgs/os-specific/linux/minimal-bootstrap/ln-boot/ln.c

···
   if (argc != 4 || strcmp(argv[1], "-s")) {
     fputs("Usage: ", stdout);
     fputs(argv[0], stdout);
-    fputs("ln -s TARGET LINK_NAME\n", stdout);
+    fputs(" -s TARGET LINK_NAME\n", stdout);
     exit(EXIT_FAILURE);
   }