Acknowledgment section title is not inserted in acm-html format

ACM template has a special acknowledgement div that automatically inserts unnumbered section header. However, the section header is not inserted in `acm-html` format.

# Reprex

Try to insert acknowledgement section by using the following div. The section title only works for `acm-pdf` format.

``` md

::: acks

We thank ...

:::

```