Boot time labs: fix bootloader instructions

Signed-off-by: Michael Opdenacker <michael.opdenacker@bootlin.com>

labs/boot-time-build-bootloader/boot-time-build-bootloader.tex

@@ -89,12 +89,15 @@ \section{Booting your new bootloader}
 \end{verbatim}
 
 You should see the \code{MLO} file in the list of files. Let's remove it
-by issuing the below command:
+by issuing the below command (erasing the first 1MiB on the eMMC):
 
 \begin{verbatim}
-fatwrite mmc 1 81000000 100000
+mmc dev 1
+mmc erase 0 100000
 \end{verbatim}
 
+You can now see that \code{fatls mmc 1} no longer sees any file.
+
 If your board doesn't boot at all, please fix it first using our instructions
 on
 \url{https://raw.githubusercontent.com/bootlin/training-materials/master/lab-data/common/bootloader/beaglebone-black/README.txt}.