Address rubocop feedback

Author: mrmargolis

lib/grape-swagger.rb

@@ -426,7 +426,7 @@ def is_primitive?(type)
             end
 
             def generate_typeref(type)
-              type = type.to_s.sub(/^[A-Z]/) {|f| f.downcase } if type.kind_of?(Class)
+              type = type.to_s.sub(/^[A-Z]/) { |f| f.downcase } if type.is_a?(Class)
               if is_primitive? type
                 { 'type' => type }
               else