Replace singular form of post type name with plural

Following [Codex description of `register_post_type` function](http://codex.wordpress.org/Function_Reference/register_post_type), base name of custom post type should be in plural form, and this is how all default post types in WordPress are registered.

> 'name' - general name for the post type, usually plural. The same as, and overridden by $post_type_object->label