fix link to custom runtimes

[kretes]

No description provided.

[adriangonz]

Good spot @kretes ! Thanks for fixing this one.

[agrski]

I don't think this PR fixes anything for how the docs are meant to be viewed -- browsing the runtimes page and clicking the relevant link for custom runtimes works fine for me.

I suspect this is about the docs not linking correctly when browsing in GitHub, but that's a suboptimal experience in any case, as this is a Sphinx project rather than pure Markdown and various things don't render correctly.

With that sald, I'll check this doesn't break anything, as I think Sphinx is generally permissive about .md vs. .html vs. `` endings for links, although sometimes it's more particular.

[agrski]

Running docs locally, this change looks to still work, so fine from my side too