README.hardware: minor update to edgerouter section

Trivial fix

(From meta-yocto rev: 9e71ba9e5d9ce874770e0c2b5c838ec853d16a5d)

Signed-off-by: Maxin B. John <maxin.john@intel.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

README.hardware

@@ -317,7 +317,7 @@ Steps:
 
  2. Flash the image.
 
-    # dd core-image-minimal-edgerouter.wic of=/dev/sdb
+    # dd if=core-image-minimal-edgerouter.wic of=/dev/sdb
 
  3. Insert USB disk into the edgerouter and boot it.