[range.chunk.outer.value] Add missing private specifier

Author: hewillk

source/ranges.tex

@@ -11209,6 +11209,7 @@
   template<@\libconcept{view}@ V>
     requires @\libconcept{input_range}@<V>
   struct chunk_view<V>::@\exposid{outer-iterator}@::value_type : view_interface<value_type> {
+  private:
     chunk_view* @\exposid{parent_}@;                                        // \expos
 
     constexpr explicit value_type(chunk_view& parent);          // \expos