doc: remove problematic auto-linking of curl man pages

The only instance of curl man page linking is in repl.md and it is explicit. The magic autolinking in html.js creates a superfluous empty link. Remove it.

Before, two adjacent links generated, with the first one having no text:

<a href="https://curl.haxx.se/docs/manpage.html"></a>
<a href="https://curl.haxx.se/docs/manpage.html"><code>curl(1)</code></a>

After, just one link:

<a href="https://curl.haxx.se/docs/manpage.html"><code>curl(1)</code></a>

Checklist

• documentation is changed or added
• commit message follows commit guidelines