Use layer digest in the description of DiffID

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

Author: coolljt0725

config.md

@@ -32,7 +32,7 @@ This specification uses the following terms:
     <dd>
 	A layer DiffID is a SHA256 digest over the layer's uncompressed tar archive and serialized in the descriptor digest format, e.g., <code>sha256:a9561eb1b190625c9adb5a9513e72c4dedafc1cb2d4c5236c9a6957ec7dfd5a9</code>.
 	Layers must be packed and unpacked reproducibly to avoid changing the layer ID, for example by using tar-split to save the tar headers.
-	NOTE: the DiffID is different than the digest in the manifest list because the manifest digest is taken over the gzipped layer for <code>application/vnd.oci.image.layer.tar+gzip</code> types.
+	NOTE: the DiffID is different than the layer digest because the digest is taken over the content of the uncompressed layer tar for <code>application/vnd.oci.image.layer.tar+gzip</code> types.
     </dd>
     <dt>
         Layer ChainID