std.h c++23 build fix

Add ::value to is_formattable<...> as per suggestion by @vitaut in #3854

include/fmt/std.h

@@ -271,7 +271,7 @@ FMT_EXPORT
 template <typename T, typename E, typename Char>
 struct formatter<
     std::expected<T, E>, Char,
-    std::enable_if_t<is_formattable<T, Char> && is_formattable<E, Char>>> {
+    std::enable_if_t<is_formattable<T, Char>::value && is_formattable<E, Char>::value>> {
   template <typename ParseContext>
   FMT_CONSTEXPR auto parse(ParseContext& ctx) -> decltype(ctx.begin()) {
     return ctx.begin();