digest: use digest.Parse over ParseDigest

Signed-off-by: Stephen J Day <stephen.day@docker.com>

reference/reference.go

@@ -170,7 +170,7 @@ func Parse(s string) (Reference, error) {
 	}
 	if matches[3] != "" {
 		var err error
-		ref.digest, err = digest.ParseDigest(matches[3])
+		ref.digest, err = digest.Parse(matches[3])
 		if err != nil {
 			return nil, err
 		}