Mention necessary GND (ground) connection to enable flashing

The description has not been entirely clear abouth the necessity of a ground connection between the fpc connector and usb-serial device. 

While this is essential, it is not obvious from the pictures provided. This is hereby fixed.

Author: oe4dns

wiki/How-to-use-keyboard-UART-port-to-flash-firmware.md

@@ -21,10 +21,11 @@ here is the process:
 3 is TX(PA9)   
 4 is RX(PA10)  
 
-
 ## 1st pin of UART on board
 ![7130803](https://github.com/user-attachments/assets/eeed0c0a-ea86-4496-b931-dcc074835eca)
 
+Do not forget to **also connect GND** (ground) between the fpc 6p connector and the serial-usb converter.
+
 ## Flashing command
 
 ```