doc: Fix a link in the FocusSerial documentation

When merging into the monorepo, all documentation links were updated to point at
the new documentation locations. Even those where the plugin wasn't merged, such
as `Focus` (`FocusSerial` was). Restore the link, so it points where it needs to.

Signed-off-by: Gergely Nagy <algernon@keyboard.io>

doc/plugin/FocusSerial.md

@@ -4,7 +4,7 @@ Bidirectional communication for Kaleidoscope. With this plugin enabled, plugins
 
 This plugin is an upgrade of the former [Kaleidoscope-Focus][kaleidoscope:focus] plugin. See the [UPGRADING.md][upgrading] document for information about how to transition to the new system.
 
- [kaleidoscope:focus]: Focus.md
+ [kaleidoscope:focus]: https://github.com/keyboardio/Kaleidoscope-Focus
  [upgrading]: ../../UPGRADING.md#bidirectional-communication-for-plugins
 
 ## Using the plugin