Change format

Author: jcarter3

spec.md

@@ -577,7 +577,6 @@ The response MUST be a json body in the following format:
       "mediaType": "application/vnd.oci.image.manifest.v1+json",
       "size": 1234,
       "digest": "sha256:a1a1a1...",
-      "artifactType": "application/vnd.example.sbom.v1",
       "annotations": {
         "org.opencontainers.image.created": "2022-01-01T14:42:55Z",
         "org.opencontainers.image.ref.name": "<tag1>"
@@ -587,7 +586,6 @@ The response MUST be a json body in the following format:
       "mediaType": "application/vnd.oci.image.manifest.v1+json",
       "size": 1234,
       "digest": "sha256:a2a2a2...",
-      "artifactType": "application/vnd.example.signature.v1",
       "annotations": {
         "org.opencontainers.image.created": "2022-01-01T07:21:33Z",
         "org.opencontainers.image.ref.name": "<tag2>"