nixos/doc: Split mkdir mode into chmod command for clarity

As recommended by ShellCheck
<https://github.com/koalaman/shellcheck/wiki/SC2174>.

nixos/doc/manual/development/writing-modules.chapter.md

@@ -141,7 +141,8 @@ in {
         path  = [ pkgs.su ];
         script =
           ''
-            mkdir -m 0755 -p $(dirname ${toString cfg.output})
+            mkdir -p $(dirname ${toString cfg.output})
+            chmod 0755 $(dirname ${toString cfg.output})
             exec updatedb \
               --localuser=${cfg.localuser} \
               ${optionalString (!cfg.includeStore) "--prunepaths='/nix/store'"} \