[span.syn] Add index entry for dynamic_extent. (#4136)

Author: Eelis

source/containers.tex

@@ -10601,7 +10601,7 @@
 \begin{codeblock}
 namespace std {
   // constants
-  inline constexpr size_t dynamic_extent = numeric_limits<size_t>::max();
+  inline constexpr size_t @\libglobal{dynamic_extent}@ = numeric_limits<size_t>::max();
 
   // \ref{views.span}, class template span
   template<class ElementType, size_t Extent = dynamic_extent>