pico-imx6: README: Fix the boot mode settings URL

The original URL that explains the boot mode setting is no longer
valid.

Update to the new one.

Signed-off-by: Fabio Estevam <festevam@gmail.com>
diff --git a/board/technexion/pico-imx6/README b/board/technexion/pico-imx6/README
index f1e84bf..f497086 100644
--- a/board/technexion/pico-imx6/README
+++ b/board/technexion/pico-imx6/README
@@ -20,7 +20,7 @@
 go to step 2 below in order to update U-Boot.
 
 Put pico board in USB download mode (Refer to the following link for details:
-https://www.technexion.com/support/knowledgebase/boot-configuration-settings-for-pico-baseboards/).
+https://developer.technexion.com/docs/pico-evaluation-kit-boot-mode-settings )
 
 Connect a USB to serial adapter between the host PC and pico.