Embedded Linux slides: Add a busybox ubi limitation

Busybox ubidetach does not accept -m <MTD-device-id>
It accepts -d <UBI-device-id> which is one of the mtd-utils ubidetach
options.

Signed-off-by: Nasser Afshin <afshin.nasser@gmail.com>
Reviewed-by: Michael Opdenacker <michael.opdenacker@bootlin.com>

slides/sysdev-flash-filesystems/sysdev-flash-filesystems.tex

@@ -843,6 +843,8 @@ \section{Flash storage and filesystems}
     \item \code{ubirsvol} doesn't support \code{-N <name>}. You have
       to specify the volume to resize by its id (\code{-n num}):\\
       \code{ubirsvol /dev/ubi0 -n 4 -s 64 MiB}
+    \item \code{ubidetach} doesn't support \code{-m <MTD-device-id>}. You
+      have to specify the UBI device id by \code{-d <UBI-device-id>}
     \end{itemize}
 \end{frame}