Update shared_pointer.hpp

Name correction: decrement_n_destroy() to decrement_and_destroy().
There's no decrement_n_destroy(), only decrement_and_destroy() is found.

Author: flyxyz123

ch16/ex16.28/shared_pointer.hpp

@@ -142,7 +142,7 @@ namespace cp5
         {
             if (ptr != pointer)
             {
-                decrement_n_destroy();
+                decrement_and_destroy();
                 ptr = pointer;
                 ref_count = new std::size_t(1);
             }
@@ -178,4 +178,4 @@ namespace cp5
             ptr = nullptr;
         }
     };
-}//namespace
+}//namespace