sabrelite HW setup: fix path for imx_usb

Since the `usb_imx` utility is not installed into PATH we have to use
relative or absolute path for calling it.

Signed-off-by: Denis Pynkin <denis.pynkin@collabora.com>

content/reference_hardware/imx6q_sabrelite_setup.md

@@ -119,7 +119,7 @@ a USB to microUSB cable.
 
 <!-- end list -->
 
-    $ sudo imx_usb ./u-boot.imx
+    $ sudo ./imx_usb ./u-boot.imx
 
 - Board should boot U-Boot and program the flash using the image on the SD
   card.