Update spec.md

Co-authored-by: Jon Johnson <[email protected]>
Signed-off-by: Josh Dolitsky <[email protected]>

spec.md

@@ -556,7 +556,7 @@ If the request is invalid, such as a `<digest>` with an invalid syntax, a `400 B
 
 Upon success, the response MUST be a JSON body with an image index containing a list of descriptors.
 Each descriptor is of an image manifest in the same `<name>` namespace with a `subject` field that specifies the value of `<digest>`.
-The descriptors MUST include an `artifactType` field that is set to the value of the `artifactType` in the image manifest.
+The descriptors MUST include an `artifactType` field that is set to the value of the `artifactType` in the image manifest, if present.
 If the `artifactType` is empty or missing in the image manifest, the value of `artifactType` MUST be set to the config descriptor `mediaType` value.
 The descriptors MUST include annotations from the image manifest.
 If a query results in no matching referrers, an empty manifest list MUST be returned.