[optional.syn] Use decay_t<T> directly instead of "see below" (#7247)

Author: frederick-vs-ja

source/utilities.tex

@@ -3238,7 +3238,7 @@
     constexpr void swap(optional<T>&, optional<T>&) noexcept(@\seebelow@);
 
   template<class T>
-    constexpr optional<@\seebelow@> make_optional(T&&);
+    constexpr optional<decay_t<T>> make_optional(T&&);
   template<class T, class... Args>
     constexpr optional<T> make_optional(Args&&... args);
   template<class T, class U, class... Args>