The stub of AccountManager::changeUserName() should look like this:
   void AccountManager::changeUserName(const string& oldName, const string& newName) {
This method should behave as follows:
  1. Use myAccounts.find() to get an iterator it pointing to the pair containing userName and its password.
  2. If it is different from myAccounts.end():
  3. Otherwise:
To declare the iterator it, review how we declared an iterator in Part I of today's exercise. You will need to do something similar, but since myAccounts is a map, you will need to declare this iterator using a map instead of a set.