MONGOID-5539: Fix example in aggregation reference documentation (#5536)

* Update aggregation.txt

* Update aggregation.txt

Author: alexbevi

docs/reference/aggregation.txt

@@ -76,7 +76,7 @@ could do the following:
     } },
     {'$project' => {_id: 1}},
   ])
-  bands = Band.find(band_ids)
+  bands = Band.find(band_ids.to_a)
 
 Note that the aggregation pipeline, since it is implemented by the Ruby driver
 for MongoDB and not Mongoid, returns raw ``BSON::Document`` objects rather than