[const.iterators.iterator] Add \expos comment for concept

Author: hewillk

source/iterators.tex

@@ -4211,7 +4211,7 @@
 \begin{codeblock}
 namespace std {
   template<class I>
-    concept @\exposconcept{not-a-const-iterator}@ = @\seebelow@;
+    concept @\exposconcept{not-a-const-iterator}@ = @\seebelow@;                   // \expos
 
   template<@\libconcept{input_iterator}@ Iterator>
   class @\libglobal{basic_const_iterator}@ {