Fix issue #610 - odoc html-fragment not producing headings correctly

[jonludlam]

@dbuenzli could you please check this works for you? As a quick check, with this I get:

$ echo "{0:sample OCaml 4.11.1 sample package documentation}" > frag.mld 
$ dune exec -- odoc html-fragment frag.mld  
<h1 id="sample"><a href="#sample" class="anchor"></a>OCaml 4.11.1 sample package documentation</h1>

[dbuenzli]

Works as you mention.