message: docs: Fix default theme URL
diff: @@ -11,7 +11,7 @@ import BorderWidth from './components/BorderWidth';
  # Theme reference
  In order to define a new theme, you should make a copy of the
  -[default theme](https://github.com/sumup/circuit-ui/blob/master/src/themes/default.js).
  +[default theme](https://github.com/sumup/circuit-ui/blob/master/src/themes/circuit.js).
  Wondering how to make the theme accessible to your components? Read
  [Getting started for Developers](/getting-started/developers).
  