[iterator.synopsis] Fix inconsistent template constraint

Author: hewillk

source/iterators.tex

@@ -346,7 +346,7 @@
     using const_sentinel = @\seebelow@;                                               // freestanding
 
   // \ref{const.iterators.iterator}, class template \tcode{basic_const_iterator}
-  template<class Iterator>
+  template<@\libconcept{input_iterator}@ Iterator>
     class basic_const_iterator;                                                     // freestanding
 
   template<class T, @\libconcept{common_with}@<T> U>