boot/uboot: update link to the U-Boot docs

The denx.de/wiki/U-Boot link now redirects to docs.u-boot.org/en/latest
Replace the link to the new location for the U-Boot documentation

Signed-off-by: Bryan Brattlof <bb@ti.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
(cherry picked from commit 164d9f0546372eb076506eebc8f88c6c4a544a36)
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

boot/uboot/Config.in

@@ -3,7 +3,7 @@ config BR2_TARGET_UBOOT
 	help
 	  Build "Das U-Boot" Boot Monitor
 
-	  https://www.denx.de/wiki/U-Boot
+	  https://docs.u-boot.org/en/latest/
 
 if BR2_TARGET_UBOOT
 choice