[memory.syn] add default argument to owner_less

Also change class-key to 'struct' to match later definition.

Author: jwakely

source/utilities.tex

@@ -3136,7 +3136,7 @@
   template<class T> void swap(weak_ptr<T>& a, weak_ptr<T>& b) noexcept;
 
   // \ref{util.smartptr.ownerless}, class template owner_less:
-  template<class T> class owner_less;
+  template<class T = void> struct owner_less;
 
   // \ref{util.smartptr.enab}, class template enable_shared_from_this:
   template<class T> class enable_shared_from_this;