Replace detach with kill for screen

Detaching from screen still leaves it running in the background which causes people to need to use `sudo` to attach to it again.
Instead if we kill the screen instance with `k` then we can just attach screen again in unprivileged mode.
You also need to confirm the exit by pressing `y`.

README.md

@@ -516,7 +516,8 @@ screen /dev/ttyUSB1 115200
 If you see an immediate `[screen is terminating]`, it may mean that you need super user rights.
 In this case, you may try using `sudo`.
 
-To exit from the `screen` command, you should press control and a together, then release these two keys and press `d`.
+To exit from the `screen` command, you should press `ctrl-a` followed by `k`.
+You will need to confirm the exit by pressing `y`.
 
 ## Debugging an application