debugging: labs: remove -j from Buildroot make command

It's useless with Buildroot as we are not using top-level parallel build.

Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>

labs/debugging-setup/debugging-setup.tex

@@ -18,7 +18,7 @@ \section{Building the image}
 $ cd buildroot
 $ git checkout debugging-training/2022.08
 $ make stm32mp157a_dk1_debugging_defconfig
-$ make -j12
+$ make
 \end{bashinput}
 
 This will take a few minutes. At the end of the build, a images directory will