docs: use useMongooseAggCursor for aggregate docs

Fix #2955

Author: vkarpov15

lib/aggregate.js

@@ -574,7 +574,7 @@ Aggregate.prototype.option = function(value) {
  *
  * ####Example:
  *
- *     var cursor = Model.aggregate(..).cursor({ batchSize: 1000 }).exec();
+ *     var cursor = Model.aggregate(..).cursor({ batchSize: 1000, useMongooseAggCursor: true }).exec();
  *     cursor.each(function(error, doc) {
  *       // use doc
  *     });