Call clear() instead of empty() in addressbook/part6 tutorial.

empty() doesn't remove elements, it just returns whether it is empty.

Task-number: QTBUG-8849
Change-Id: I9bb8e855d96cecd2a0df336cb46e308ddedf0d07
Reviewed-by: Michalina Ziemba <michalina.ziemba@nokia.com>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

examples/tutorials/addressbook/part6/addressbook.cpp

@@ -373,7 +373,7 @@ void AddressBook::loadFromFile()
         
         QDataStream in(&file);
         in.setVersion(QDataStream::Qt_4_5);
-        contacts.empty();   // empty existing contacts
+        contacts.clear();   // clear existing contacts
         in >> contacts;
 //! [loadFromFile() function part2]