[Docs] Fix heading in composite-aggregation.asciidoc
The heading for the "Missing buckets" should be on the same level
as the the "Order" section.
@@ -348,7 +348,7 @@ GET /_search
\... will sort the composite bucket in descending order when comparing values from the `date_histogram` source
and in ascending order when comparing values from the `terms` source.
-====== Missing bucket
+==== Missing bucket
By default documents without a value for a given source are ignored.
It is possible to include them in the response by setting `missing_bucket` to
