[libc++][NFC] Fix typo in ranges::iterator_t synopsis

The `iterator_t` alias template is on `T` not a `R` like the other
neighboring alias templates. Fix the typo.

diff --git a/libcxx/include/ranges b/libcxx/include/ranges
index 8a99ee6..dd7decf 100644 (file)
--- a/libcxx/include/ranges
+++ b/libcxx/include/ranges
@@ -36,7 +36,7 @@ namespace std::ranges {
     inline constexpr bool enable_borrowed_range = false;
 
   template<class T>
-    using iterator_t = decltype(ranges::begin(declval<R&>()));
+    using iterator_t = decltype(ranges::begin(declval<T&>()));
   template<range R>
     using sentinel_t = decltype(ranges::end(declval<R&>()));
   template<range R>