Improve example of quick install in README

There's really no reason to use the resource-like declaration when
you're not specifying any class parameters. It's cleaner and safer to
use `include`.

README.md

@@ -29,7 +29,7 @@ This module manages NGINX configuration.
 ### Install and bootstrap an NGINX instance
 
 ```puppet
-class { 'nginx': }
+include nginx
 ```
 
 ### A simple reverse proxy