Kernel labs: use a github.com link

People in the training (including myself) had a hard time finding the
pre-compiled binaries because linking to the raw README.txt file
points to a raw.githubusercontent.com link, from which it is
"difficult" to find the binaries listed in the file. Point to the
github.com page instead, people will find out the README.TXT by
themselves (it is displayed on the webpage pointed anyway).

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>

labs/kernel-board-setup/kernel-board-setup.tex

@@ -125,8 +125,8 @@ \section{Bootloader interaction}
 
 If you don't have this command, it's probably because you are doing these labs on your own
 (i.e. without participating to a Bootlin course), we ask you to install the U-Boot binary
-that we compiled and tested. See instructions in
-\url{https://raw.githubusercontent.com/bootlin/training-materials/master/lab-data/common/bootloader/beaglebone-black/README.txt}
+that we compiled and tested. See instructions at
+\url{https://github.com/bootlin/training-materials/tree/master/lab-data/common/bootloader/beaglebone-black}
 for a simple way to do this.
 
 To avoid trouble because of settings applied in previous practical labs,