Correct the path of zephyr sample dts file

frdm_k64f.dts has been moved to boards/arm/frdm_k64f/

Signed-off-by: Dong Xiang <dong.xiang@linaro.org>
diff --git a/README-zephyr.rst b/README-zephyr.rst
index 3d86165..e2a6186 100644
--- a/README-zephyr.rst
+++ b/README-zephyr.rst
@@ -23,8 +23,8 @@
 so your SoC runs it out of reset.
 
 An example DTS file with flash partitions defined is in the Zephyr
-file dts/arm/frdm_k64f.dts. Make sure the labels in your board's DTS
-match the ones used there.
+file boards/arm/frdm_k64f/frdm_k64f.dts. Make sure the labels in your
+board's DTS match the ones used there.
 
 Building the bootloader itself
 ==============================