Docs: heading level in compose aggs is too high

[0bsearch]

Elasticsearch version: 6.3

Steps to reproduce:

1.Go to https://www.elastic.co/guide/en/elasticsearch/reference/current/search-aggregations-bucket-composite-aggregation.html#_date_histogram
2. See formatting for Format/Time Zone sections

Formatting in source is ======, which is higher than default asciidoc limit (and, I guess, higher than current pre-processor settings).

[elasticmachine]

Pinging @elastic/es-search-aggs