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>