10-serial-communication/nix-tooling.md /dev/ttyUSB0 permissions

Correct the /dev/ttyUSB0 permissions, and add note about udev rules

Author: rj-xy

src/10-serial-communication/nix-tooling.md

@@ -16,9 +16,12 @@ But what's this `ttyUSB0` thing? It's a file of course! Everything is a file in
 
 ``` console
 $ ls -l /dev/ttyUSB0
-crw-rw---- 1 root uucp 188, 0 Oct 27 00:00 /dev/ttyUSB0
+crw-rw-rw- 1 root uucp 188, 0 Oct 27 00:00 /dev/ttyUSB0
 ```
 
+> **NOTE** if the permissions above is `crw-rw----`, the udev rules have not been set correctly
+> see [udev rules](../03-setup/linux.html#udev-rules)
+
 You can send out data by simply writing to this file:
 
 ``` console