media-types: Replace Docker link with internal descriptor link

The Docker link landed in 7b39189 (media-types: show relations
between media types, 2016-05-02, #55), but we've had a descriptor
definition locally since d49d055 (spec: describe descriptors and
digests, 2016-06-01, #111).

Signed-off-by: W. Trevor King <wking@tremily.us>

media-types.md

@@ -45,5 +45,5 @@ The following figure shows how the above media types reference each other:
 
 ![](img/media-types.png)
 
-A reference is defined as the target content digest, as defined by the [Registry V2 HTTP API Specificiation](https://docs.docker.com/registry/spec/api/#digest-parameter).
+[Descriptors](descriptor.md) are used for all references.
 The manifest list being a "fat manifest" references one or more image manifests per target platform. An image manifest references exactly one target configuration and possibly many layers.