[range.stride.iterator] Add missing \exposid

Author: tkoeppe

source/ranges.tex

@@ -13760,7 +13760,7 @@
     using iterator_concept = @\seebelow@;
     using iterator_category = @\seebelow@;    // not always present
 
-    iterator() requires @\libconcept{default_initializable}@<iterator_t<@\exposid{Base}@>> = default;
+    @\exposid{iterator}@() requires @\libconcept{default_initializable}@<iterator_t<@\exposid{Base}@>> = default;
 
     constexpr @\exposid{iterator}@(@\exposid{iterator}@<!Const> other)
       requires Const && @\libconcept{convertible_to}@<iterator_t<V>, iterator_t<@\exposid{Base}@>>