Fix formatting in update-by-query (#22628)

The automatic slicing section wasn't displaying the bullet list correctly.

docs/reference/docs/update-by-query.asciidoc

@@ -501,6 +501,7 @@ Which results in a sensible `total` like this one:
 
 Adding `slices` to `_update_by_query` just automates the manual process used in
 the section above, creating sub-requests which means it has some quirks:
+
 * You can see these requests in the
 <<docs-update-by-query-task-api,Tasks APIs>>. These sub-requests are "child"
 tasks of the task for the request with `slices`.