[execution.syn] Add \libconcept for sender_in (#7182)

Author: hewillk

source/exec.tex

@@ -540,7 +540,7 @@
   template<class Sndr, class Env = empty_env,
            template<class...> class Tuple = @\exposid{decayed-tuple}@,
            template<class...> class Variant = @\exposid{variant-or-empty}@>
-      requires sender_in<Sndr, Env>
+      requires @\libconcept{sender_in}@<Sndr, Env>
     using value_types_of_t = @\seebelow@;
 
   template<class Sndr, class Env = empty_env,