manual: Fix BR2_GLOBAL_PATCH_DIR example

As discussed on irc, there is a missing part in the BR2_GLOBAL_PATCH_DIR
    example compared to the ASCII figure under customize-packages section.

Signed-off-by: Sebastien Roy <sebastien.roy@savoirfairelinux.com>
Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>