Move dh:changeKind under dh:Change for commits

[xdrcft8000]

Currently the JSON response for a commit is:

{
  "dh:updates": "09bafe350c94f417e9a881dd4d20e518cb9ade9b9c042ff7bbc3e48aaaa69832",
  "dh:revisionSnapshotCSV": "032b95eb3577d01e3dadca20336563ef2f1f4b1483747183e5130c2eadeabce4",
  "dcterms:description": "change for x-series-on-GCP",
  "dh:appliesToRevision": "https://example.org/data/x-series-on-GCP/release/release-1/revision/1",
  "dcterms:format": "text/csv",
  "@id": "x-series-on-GCP/release/release-1/revision/1/commit/1",
  "@type": "dh:Change",
  "dh:changeKind": "https://publishmydata.com/def/datahost/ChangeKindAppend",
  "@context": [
    "https://cdn.jsdelivr.net/gh/Swirrl/datahost-prototypes@1282114/datahost-ld-openapi/resources/jsonld-context.json",
    {
      "@base": "https://example.org/data/"
    }
  ]
}

dh:changeKind should instead be under @type, where "dh:changeKind" is a subclass of "dh:Change"