Update aggregation.asciidoc
There are four kinds of aggregations now, not three.
@@ -11,7 +11,7 @@ the execution defines what this document set is (e.g. a top-level aggregation ex
query/filters of the search request).
There are many different types of aggregations, each with its own purpose and output. To better understand these types,
-it is often easier to break them into three main families:
+it is often easier to break them into four main families:
<<search-aggregations-bucket, _Bucketing_>>::
A family of aggregations that build buckets, where each bucket is associated with a _key_ and a document
