At no point on this page does it mention going into the /maintenance/ directory.
Might I suggest that either that be added as a step, or the code examples be changed to include that in the PHP filename, so it can be executed from the document root?
Topic on Manual talk:ChangePassword.php/Flow
The location is stated in the info box at the top. However to make it clearer I added a note about the location from which the examples work. Thanks for your suggestion.