system/activation: mention deps attr in activationScripts example

As it helps making deps easier to discover - as we don't currently
render submodule options in the module correctly - and is arguably
more technical correct: When using nixos-install to install nixos
into a chroot in i.e. /mnt, there's no gurantee that /mnt/dev exists
before the specialfs phase ran.

nixos/modules/system/activation/activation-script.nix

@@ -110,7 +110,11 @@ in
       default = {};
 
       example = literalExpression ''
-        { stdio.text =
+        {
+          stdio = {
+            # Run after /dev has been mounted
+            deps = [ "specialfs" ];
+            text =
               '''
                 # Needed by some programs.
                 ln -sfn /proc/self/fd /dev/fd
@@ -118,6 +122,7 @@ in
                 ln -sfn /proc/self/fd/1 /dev/stdout
                 ln -sfn /proc/self/fd/2 /dev/stderr
               ''';
+          };
         }
       '';