Use layer digest in the description of DiffID

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

Author: coolljt0725

config.md

@@ -26,7 +26,7 @@ Changing it means creating a new derived image, instead of changing the existing
 A layer DiffID is a SHA256 digest over the layer's uncompressed tar archive and serialized in the descriptor digest format, e.g., `sha256:a9561eb1b190625c9adb5a9513e72c4dedafc1cb2d4c5236c9a6957ec7dfd5a9`.
 Layers must be packed and unpacked reproducibly to avoid changing the layer DiffID, 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 `application/vnd.oci.image.layer.tar+gzip` types.
+NOTE: the DiffID is different than the layer digest because it is taken over the uncompressed content of the gzipped layer for `application/vnd.oci.image.layer.tar+gzip` types.
 
 ### Layer ChainID