zephyr: Fix an invalid url in file CMakeLists.txt

Replace https://www.zephyrproject.org/doc/dts/device_tree.html with
http://docs.zephyrproject.org/devices/dts/device_tree.html

Signed-off-by: Ding Tao <miyatsu@qq.com>
diff --git a/samples/zephyr/hello-world/CMakeLists.txt b/samples/zephyr/hello-world/CMakeLists.txt
index d4ef940..ec01332 100644
--- a/samples/zephyr/hello-world/CMakeLists.txt
+++ b/samples/zephyr/hello-world/CMakeLists.txt
@@ -33,7 +33,7 @@
 # this with a device tree overlay file.
 #
 # See the Zephyr documentation for more information on DT:
-# https://www.zephyrproject.org/doc/dts/device_tree.html
+# http://docs.zephyrproject.org/devices/dts/device_tree.html
 set(DTC_OVERLAY_FILE "${CMAKE_CURRENT_SOURCE_DIR}/dts.overlay")
 
 # Standard Zephyr application boilerplate.