u-boot: fix CUSTOM_PATCH_DIR documentation

We're using uboot-<version>-*.patch, and not u-boot-<version>-*.patch

Reported-by: Jerzy Grzegorek <jerzy.grzegorek@trzebnica.net>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>

boot/uboot/Config.in

@@ -61,7 +61,7 @@ config BR2_TARGET_UBOOT_CUSTOM_PATCH_DIR
 	help
 	  If your board requires custom patches, add the path to the
 	  directory containing the patches here. The patches must be
-	  named u-boot-<version>-<something>.patch.
+	  named uboot-<version>-<something>.patch.
 
 	  Most users may leave this empty