[util.smartptr.atomic.general] Simplify example code (#6077)

Removes the redundant explicit construction of the return value,
and uses an explicit return type instead of "auto" for clarity.

Author: hewillk

source/threads.tex

@@ -4867,12 +4867,12 @@
   atomic<shared_ptr<node>> head;
 
 public:
-  auto find(T t) const {
+  shared_ptr<node> find(T t) const {
     auto p = head.load();
     while (p && p->t != t)
       p = p->next;
 
-    return shared_ptr<node>(move(p));
+    return p;
   }
 
   void push_front(T t) {