HTML "tags" not escaped in code blocks

[pietroalbini]

While reading RFD 308 (oxidecomputer/rfd#535), I noticed that the code blocks in section 7 are rendered incorrectly:

Asciidoctor	RFD website

Looking at the HTML, it seems like it's treating the placeholders as HTML tags (thanks @augustuswm for pointing this out!):

<div class="content"><pre class="highlight !block">MPN1:<mfg>:<pn>:<rev>:<serial></serial></rev></pn></mfg></pre></div>