[ranges.syn] Add ref_view to header synopsis.

Author: jensmaurer

source/ranges.tex

@@ -149,6 +149,10 @@
   template<ViewableRange R>
     using all_view = decltype(view::all(declval<R>()));
 
+  template<Range R>
+    requires is_object_v<R>
+  class ref_view;
+
   // \ref{range.filter}, filter view
   template<InputRange V, IndirectUnaryPredicate<iterator_t<V>> Pred>
     requires View<V> && is_object_v<Pred>