[util.smartptr.shared.cmp] Fix missing right parenthesis

Fixes NB JP 025 (C++23 DIS).

Author: jensmaurer

source/memory.tex

@@ -4311,7 +4311,7 @@
 \pnum
 \returns
 \begin{codeblock}
-compare_three_way()(a.get(), static_cast<typename shared_ptr<T>::element_type*>(nullptr).
+compare_three_way()(a.get(), static_cast<typename shared_ptr<T>::element_type*>(nullptr))
 \end{codeblock}
 \end{itemdescr}