docs(aggregate): fix syntax highlighting on allowDiskUse() docs

Re: #7021

lib/aggregate.js

@@ -683,7 +683,7 @@ Aggregate.prototype.explain = function(callback) {
  *
  * ####Example:
  *
- *     Model.aggregate(..).allowDiskUse(true).exec(callback)
+ *     await Model.aggregate([{ $match: { foo: 'bar' } }]).allowDiskUse(true);
  *
  * @param {Boolean} value Should tell server it can use hard drive to store data during aggregation.
  * @param {Array} [tags] optional tags for this query