[util.smartptr.atomic.general] Fix cross-reference to shared_ptr (#5222)

Author: jwakely

source/atomics.tex

@@ -2734,7 +2734,7 @@
 
 \pnum
 The library provides partial specializations of the \tcode{atomic} template
-for shared-ownership smart pointers\iref{smartptr}.
+for shared-ownership smart pointers\iref{util.sharedptr}.
 \begin{note}
 The partial specializations are declared in header \libheaderref{memory}.
 \end{note}