[exec.spawn.future] Add missing \exposid.

Misapplication of LWG Motion 2, Issue LWG 4540.

Addresses editorial review committee feedback.

Author: tkoeppe

source/exec.tex

@@ -5765,7 +5765,7 @@
       }, std::move(tuple));
     }
   });
-destroy();
+@\exposid{destroy}();
 \end{codeblock}
 \end{itemize}
 \end{itemdescr}