oci2disk: fix: update readme to new oras (#162)

## Description

This pr updates the readme of oci2disk to use the updates oras command. oras removed the `--manifest-config` flag and the default media type is [expected](https://github.com/tinkerbell/actions/blob/66e4fc9a20d946bc217ada425717e20cb9570657/oci2disk/image/image.go#L60-L61).

Fixes: #

## How Has This Been Tested?





## How are existing users impacted? What migration steps/scripts do we need?





## Checklist:

I have:

- [x] updated the documentation and/or roadmap (if required)
- [ ] added unit or e2e tests
- [ ] provided instructions on how to upgrade

Author: mergify[bot]

oci2disk/README.md

@@ -14,7 +14,8 @@ as this will simplify the process of creating a new "artifact" that can be used
 The below example will push a `debian` image to a registry:
 
 ```
-oras push  192.168.0.173/test/debian:raw.gz --manifest-config /dev/null:application/vnd.acme.rocket.config ./debian.raw.gz --insecure
+# defaults to expected layer media-type of application/vnd.oci.image.layer.v1.tar
+oras push 192.168.0.173/test/debian:raw.gz ./debian.raw.gz --insecure
 ```
 
 We can then use this image by referring too it with teh `IMG_URL` environment variable.