[allocator.uses.construction] Give a name to generic lambda type

parameter to allow idiomatic use of std::forward.

source/utilities.tex

@@ -7614,8 +7614,8 @@
 \effects
 Equivalent to:
 \begin{codeblock}
-return apply([&](auto&&...xs) {
-       return construct_at(p, std::forward<decltype(xs)>(xs)...);
+return apply([&]<class... U>(U&&... xs) {
+       return construct_at(p, std::forward<U>(xs)...);
      }, uses_allocator_construction_args<T>(alloc, std::forward<Args>(args)...));
 \end{codeblock}
 \end{itemdescr}