[const.iterators.types] Avoid "defining" iterator_category

Use "declare" instead.

Author: frederick-vs-ja

source/iterators.tex

@@ -4396,7 +4396,7 @@
 \end{itemize}
 
 \pnum
-The member \grammarterm{typedef-name} \tcode{iterator_category} is defined
+The member \grammarterm{typedef-name} \tcode{iterator_category} is declared
 if and only if \tcode{Iterator} models \libconcept{forward_iterator}.
 In that case,
 \tcode{basic_const_iterator<Iterator>::iterator_category} denotes