[exec.snd.expos] Add italics for see below

Author: hewillk

source/exec.tex

@@ -1561,7 +1561,7 @@
 \end{itemdescr}
 
 \begin{itemdecl}
-template<class Tag, class Data = see below, class... Child>
+template<class Tag, class Data = @\seebelow@, class... Child>
   constexpr auto @\exposid{make-sender}@(Tag tag, Data&& data, Child&&... child);
 \end{itemdecl}