yocto: slides: Use ${BPN} instead of ${PN} in the sourceforge example

Using ${PN} in SRC_URI now triggers a warning since it can contain
special suffixes like -native. Use ${BPN} instead which is the now
recommended way.

Signed-off-by: Maxime Chevallier <maxime.chevallier@bootlin.com>

slides/yocto-recipe-basics/yocto-recipe-basics.tex

@@ -139,7 +139,7 @@ \subsection{Organization of a recipe}
           locations: \code{${SOURCEFORGE_MIRROR}},
           \code{${GNU_MIRROR}}, \code{${KERNELORG_MIRROR}}\dots
         \item Example:
-          \code{${SOURCEFORGE_MIRROR}/<project-name>/${PN}-${PV}.tar.gz}
+          \code{${SOURCEFORGE_MIRROR}/<project-name>/${BPN}-${PV}.tar.gz}
         \item See \code{meta/conf/bitbake.conf}
       \end{itemize}
   \end{itemize}