LWG2684 priority_queue lacking comparator typedef

Replaced typedef with using as per:
[18-30] Replace typedefs with alias-declarations (#704)

Author: Dawn Perchik

source/containers.tex

@@ -8592,6 +8592,7 @@
     using const_reference = typename Container::const_reference;
     using size_type       = typename Container::size_type;
     using container_type  = Container;
+    using value_compare   = Compare;
 
   protected:
     Container c;