README updated: cdnjs link - v2.2.0 to v2.2.1

Author: lo kesh

README.md

@@ -21,7 +21,7 @@ Add `pretty.min.css` in your html
 
 From CDN,
 ```html
-<link rel="stylesheet" href="https://cdnjs.cloudflare.com/ajax/libs/pretty-checkbox/2.2.0/pretty.min.css"/>
+<link rel="stylesheet" href="https://cdnjs.cloudflare.com/ajax/libs/pretty-checkbox/2.2.1/pretty.min.css"/>
 ```
 or from the source,
 ```html