Update data_stream.yml with top level type: group (#2414)

Fix generator error by adding the "type: group" top level field to schemas/data_stream.yml

schemas/data_stream.yml

@@ -31,6 +31,7 @@
     An Elasticsearch data stream consists of one or more backing indices, and a data stream name forms part of the backing indices names.
     Due to this convention, data streams must also follow index naming restrictions. For example, data stream names cannot include `\`, `/`, `*`, `?`, `"`, `<`, `>`, `|`, ` ` (space character), `,`, or `#`.
     Please see the Elasticsearch reference for additional https://www.elastic.co/guide/en/elasticsearch/reference/current/indices-create-index.html#indices-create-api-path-params[restrictions].
+  type: group
   fields:
 
     - name: type