Added accidentally omitted "enum class refresh_rate;" declaration to "Header <experimental/io2d> synopsis".

Author: mikebmcl

source/experimental-io2d.tex

@@ -30,6 +30,7 @@
   enum class subpixel_order;
   enum class path_data_type;
   enum class scaling;
+  enum class refresh_rate;
 
   class io2d_error_category;
   const error_category& io2d_category() noexcept;