[mem.res.syn] Add default template argument for polymorphic_allocator

Fixes NB JP 008 (C++20 DIS)

Author: jensmaurer

source/utilities.tex

@@ -11454,7 +11454,7 @@
   bool operator==(const memory_resource& a, const memory_resource& b) noexcept;
 
   // \ref{mem.poly.allocator.class}, class template \tcode{polymorphic_allocator}
-  template<class Tp> class polymorphic_allocator;
+  template<class Tp = byte> class polymorphic_allocator;
 
   template<class T1, class T2>
     bool operator==(const polymorphic_allocator<T1>& a,