fix broken link to additional CSS

[nishchalatattve]

I found a weird angle bracket when reading through the docs. Maybe it was intended to be a link?

[alexvoss]

Yes, that's a missing link. We avoid putting the URLs into the text as it makes it pretty unreadable. Can you move it to a separate link definition like so:

[additional style sheet]: https://zensical.org/docs/customization#additional-css

That is, remove what you added in your commit and add the above line underneath the paragraph instead.

[nishchalatattve]

Apologies for the style. I have reformated it as requested in b051c05.

[alexvoss]

Thanks for spotting this and helping fix it!

[squidfunk]

Please use relative links to Markdown files when linking to pages inside the same project – don't specify the URL.