Fixup: add comma for readability

Author: eisenwave

source/exec.tex

@@ -2857,7 +2857,7 @@
 Otherwise, \tcode{\exposid{connect-awaitable}(new_sndr, rcvr)}.
 \end{itemize}
 Except that \tcode{rcvr} is evaluated only once.
-The program is ill-formed, no diagnostic required
+The program is ill-formed, no diagnostic required,
 if there exists an rvalue expression \tcode{rcvr2} such that:
 \begin{itemize}
 \item \tcode{decltype(rcvr2)} models \libconcept{receiver},