Fix `searchable` for `citation_finder` in `document`

app/models/cdao/document.rb

@@ -77,6 +77,8 @@ class Cdao::Document < Cdao::Base
       is_only_in_premium_libraries?
     end
 
-    string :citation_finders_names, multiple: true
+    string :citation_finders_names, multiple: true do
+      citation_finders_names
+    end
   end
 end