manual: fix uboot subdirectory in example layout

If U-Boot patches are nested under BR2_GLOBAL_PATCH_DIR, then the
subdirectory should be called "uboot", and not "u-boot".

Signed-off-by: Simon Dawson <spdawson@gmail.com>
Acked-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

docs/manual/customize-packages.txt

@@ -73,13 +73,13 @@ board/
 |       +-- linux
 |       |   +-- linux-patch1.patch
 |       |   +-- linux-patch2.patch
-|       +-- u-boot
+|       +-- uboot
 |       +-- foopkg
 +-- fooarch-board
     +-- patches
         +-- linux
         |   +-- linux-patch3.patch
-        +-- u-boot
+        +-- uboot
         +-- foopkg
 -----