[func.wrap.ref.ctor] Restore class T in constant_wrapper constructor head (#9144)

Fixes a misapplication of P3948R1 (from LWG Motion 16 of the Croydon, 2026 meeting).

Author: abhinavagarwal07

source/utilities.tex

@@ -15804,7 +15804,7 @@
 
 \indexlibraryctor{function_ref}%
 \begin{itemdecl}
-template<auto c, class F>
+template<auto c, class F, class T>
   constexpr function_ref(constant_wrapper<c, F> f, @\cv{}@ T* obj) noexcept;
 \end{itemdecl}