[exec.then] Insert missing space between words (#7511)

Author: Andreas-Krug

source/exec.tex

@@ -3373,7 +3373,7 @@
 
 \pnum
 The expression \tcode{\exposid{then-cpo}(sndr, f)} has undefined behavior
-unless it returns a sender\tcode{out_sndr} that
+unless it returns a sender \tcode{out_sndr} that
 \begin{itemize}
 \item
 invokes \tcode{f} or a copy of such