fix(logsToEs): Fixed ES '_type' attribute

BREAKING CHANGE: The '_type' attribute is now hardcoded to
'serverless-es-logs' instead of the name of the Cloudwatch
log group.

For any users of Elasticsearch 6.x this change will result
in an error for the current index.

Closes: #65

templates/code/logsToEs.js

@@ -77,7 +77,7 @@ function transform(payload) {
 
         var action = { "index": {} };
         action.index._index = indexName;
-        action.index._type = payload.logGroup;
+        action.index._type = 'serverless-es-logs';
         action.index._id = logEvent.id;
 
         bulkRequestBody += [