Update select2.rb

Author: Davor Bagarić

lib/activeadmin/select2.rb

@@ -2,4 +2,4 @@
 require 'activeadmin/select2/filter_select_input_extension.rb'
 require 'activeadmin/inputs/filter_select2_multiple_input.rb'
 
-ActiveAdmin::Inputs::Filter::SelectInput.send :include, ActiveAdmin::Select2::Inputs::FilterSelectInputExtension
+ActiveAdmin::Inputs::Filters::SelectInput.send :include, ActiveAdmin::Select2::Inputs::FilterSelectInputExtension