[exec.getcomplsigs] Fix misplaced \end{itemdescr} (#8301)

Fixes NB US 217-359 (C++26 CD).

Author: jensmaurer

source/exec.tex

@@ -2700,14 +2700,14 @@
 Otherwise,
 \tcode{(throw $except$, completion_signatures())}.
 \end{itemize}
+\end{itemdescr}
 
 \pnum
 Given a type \tcode{Env}, if
 \tcode{completion_signatures_of_t<Sndr>} and
 \tcode{completion_signatures_of_t<Sndr, Env>}
 are both well-formed,
 they shall denote the same type.
-\end{itemdescr}
 
 \pnum
 Let \tcode{rcvr} be an rvalue