Make clear about the order

Signed-off-by: Lei Jitang <[email protected]>

Author: coolljt0725

config.md

@@ -244,7 +244,7 @@ This specification uses the following terms:
             Implementations MUST generate an error if they encounter a unknown value while verifying or unpacking an image.
           </li>
           <li>
-            <code>diff_ids</code> is an array of layer content hashes (<code>DiffIDs</code>), in order from bottom-most to top-most.
+            <code>diff_ids</code> is an array of layer content hashes (<code>DiffIDs</code>), in order from bottom-most to top-most and bottom-most at index 0.
           </li>
         </ul>