LWG4455 Add missing constraint to basic-sender::get_completion_signatures definition

Fixes US 215-356 (C++26 CD).

Author: burblebee

source/exec.tex

@@ -2002,7 +2002,7 @@
 \indexlibrarymember{get_completion_signatures}{\exposid{basic-sender}}%
 \begin{itemdecl}
 template<class Tag, class Data, class... Child>
-  template<class Sndr, class... Env>
+  template<@\exposconcept{decays-to}@<@\exposid{basic-sender}@> Sndr, class... Env>
     constexpr auto @\exposid{basic-sender}@<Tag, Data, Child...>::get_completion_signatures();
 \end{itemdecl}