[execpol.unseq] Fix missing \itemdescr (#5931)

Author: onihusube

source/utilities.tex

@@ -15115,6 +15115,7 @@
 class execution::unsequenced_policy { @\unspec@ };
 \end{itemdecl}
 
+\begin{itemdescr}
 \pnum
 The class \tcode{unsequenced_policy} is an execution policy type
 used as a unique type to disambiguate parallel algorithm overloading and
@@ -15127,6 +15128,7 @@
 the \tcode{execution::unsequenced_policy} policy,
 if the invocation of an element access function exits via an exception,
 \tcode{terminate} is invoked\iref{except.terminate}.
+\end{itemdescr}
 
 \rSec2[execpol.objects]{Execution policy objects}