Fix inconsistent layer media type

Change layer media type for consistency with `media-types.md`, add
`.v1` to layer media type.

Signed-off-by: Zhang Wei <[email protected]>

Author: WeiZhang555

layer.md

@@ -4,7 +4,7 @@ This document describes how to serialize a filesystem and filesystem changes lik
 One or more layers are applied on top of each other to create a complete filesystem.
 This document will use a concrete example to illustrate how to create and consume these filesystem layers.
 
-This section defines the `application/vnd.oci.image.layer.tar+gzip` and `application/vnd.oci.image.layer.nondistributable.tar+gzip` [media types](media-types.md).
+This section defines the `application/vnd.oci.image.layer.v1.tar+gzip` and `application/vnd.oci.image.layer.nondistributable.v1.tar+gzip` [media types](media-types.md).
 
 ## Distributable Format