docs: index: Reword GH Discussions link
<h2>Help</h2>
<p>
Join <code>#isso</code> on <a href="https://libera.chat/">Libera.Chat</a>
- or open an issue on <a href="https://github.com/posativ/isso/discussions">GitHub</a>.
+ or ask a question on <a href="https://github.com/posativ/isso/discussions">GitHub Discussions</a>.
</p>
<h2>Contribute</h2>
<p>
