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.

Author: phaer

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

@@ -110,14 +110,19 @@ in
       default = {};
 
       example = literalExpression ''
-        { stdio.text =
-          '''
-            # Needed by some programs.
-            ln -sfn /proc/self/fd /dev/fd
-            ln -sfn /proc/self/fd/0 /dev/stdin
-            ln -sfn /proc/self/fd/1 /dev/stdout
-            ln -sfn /proc/self/fd/2 /dev/stderr
-          ''';
+        {
+          stdio = {
+            # Run after /dev has been mounted
+            deps = [ "specialfs" ];
+            text =
+              '''
+                # Needed by some programs.
+                ln -sfn /proc/self/fd /dev/fd
+                ln -sfn /proc/self/fd/0 /dev/stdin
+                ln -sfn /proc/self/fd/1 /dev/stdout
+                ln -sfn /proc/self/fd/2 /dev/stderr
+              ''';
+          };
         }
       '';