manual: fix missing DL_DIR rename to BR2_DL_DIR

There was one instance of DL_DIR that was left out. Fix that.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Samuel Martin <s.martin49@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

docs/manual/adding-packages-generic.txt

@@ -253,7 +253,7 @@ information is (assuming the package name is +libfoo+) :
   Buildroot should download from +LIBFOO_SITE+ in addition to the main
   +LIBFOO_SOURCE+ (which usually is a tarball). Buildroot will not do
   anything with those additional files, except download files: it will
-  be up to the package recipe to use them from +$(DL_DIR)+.
+  be up to the package recipe to use them from +$(BR2_DL_DIR)+.
 
 * +LIBFOO_SITE_METHOD+ determines the method used to fetch or copy the
   package source code. In many cases, Buildroot guesses the method