Fix Frontends link in readme
In Markdown, at least for the GitHub implementation, title anchors
are rendered in lowercase.
As such the "Frontends" link on the note didn't jump to the list.
<h4 align="center">A modern editor with a backend written in Rust.</h4>
-***Note:*** *This is repo contains only the editor core, which is not usable on its own. For editors based on it, check out the list in [Frontends](#Frontends).*
+***Note:*** *This is repo contains only the editor core, which is not usable on its own. For editors based on it, check out the list in [Frontends](#frontends).*
The xi-editor project is an attempt to build a high quality text editor,
using modern software engineering techniques. It is initially built for
