Docs: fold section into current chapter

In #25602 we added a new *chapter* on aggregating by day of the
week. We intended to add a new *section* but we were missing a
single `=`.

docs/reference/aggregations/bucket/datehistogram-aggregation.asciidoc

@@ -398,7 +398,7 @@ the `order` setting. Supports the same `order` functionality as the <<search-agg
 
 deprecated[6.0.0, Use `_key` instead of `_time` to order buckets by their dates/keys]
 
-=== Use of a script to aggregate by day of the week
+==== Use of a script to aggregate by day of the week
 
 There are some cases where date histogram can't help us, like for example, when we need
 to aggregate the results by day of the week.
@@ -451,4 +451,4 @@ Response:
 // TESTRESPONSE[s/\.\.\./"took": $body.took,"timed_out": false,"_shards": $body._shards,"hits": $body.hits,/]
 
 The response will contain all the buckets having as key the relative day of
-the week: 1 for Monday, 2 for Tuesday... 7 for Sunday.
+the week: 1 for Monday, 2 for Tuesday... 7 for Sunday.