QEMU labs: use the [Ctrl][a] [x] trick to exit QEMU

Signed-off-by: Michael Opdenacker <michael.opdenacker@bootlin.com>
Reported-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>

labs/sysdev-u-boot-qemu/sysdev-u-boot-qemu.tex

@@ -91,11 +91,9 @@ \section{Testing U-Boot}
 You can then type the \code{help} command, and explore the few commands
 available.
 
-Note: to exit QEMU, run the below command in another terminal:
-
-\begin{verbatim}
-killall qemu-system-arm
-\end{verbatim}
+Note: to exit QEMU, type \code{[Ctrl][a]} followed by \code{[h]}
+to see available commands. One of them is \code{[Ctrl][a]} followed by
+\code{[x]}, which allows to exit the emulator.
 
 \section{SD card setup}