html-fragment turns headings into paragraphs

[dbuenzli]

> echo "{0:sample OCaml 4.11.1 sample package documentation}" > frag.mld
> odoc html-fragment frag.mld
<p>OCaml 4.11.1 sample package documentation</p>

This breaks the odig odoc --index-intro option used e.g. by https://docs.mirage.io/, https://b0-system.github.io/odig/doc/, https://ocaml.janestreet.com/ocaml-core/latest/doc/index.html or https://binaryanalysisplatform.github.io/bap/api/master/index.html

[dbuenzli]

Still an issue 3.0.0~beta1.