fix broken button

docs/4.0/getting-started/download.md

@@ -12,7 +12,7 @@ toc: true
 
 **Current version:** v{{ site.current_version}}
 
-<a href="{{ site.download.dist }}" class="btn btn-lg " onclick="ga('send', 'event', 'Getting started', 'Download', 'Download Bootstrap');">Download Bootstrap</a>
+<a href="{{ site.download.dist }}" class="btn btn-bd-purple" onclick="ga('send', 'event', 'Getting started', 'Download', 'Download Bootstrap');">Download Bootstrap</a>
 
 ## Source files
 **Want to compile Bootstrap with your project's asset pipeline?** Choose this option to download our source Sass, JavaScript, and documentation files. Requires a Sass compiler, [Autoprefixer](https://github.com/postcss/autoprefixer), [postcss-flexbugs-fixes](https://github.com/luisrudge/postcss-flexbugs-fixes), and [some setup]({{ site.baseurl }}/docs/{{ site.docs_version }}/getting-started/build-tools/#tooling-setup).