Corrects a typo in the tests/test_hooks.py.

In `test_git_hook_uses_the_configuration_file_specified_in_settings_path`,
`modified_file_path` incorrectly pointed to a file that was a sibling of the
configuration file, such that the configuration file would have been found
indepently of the `settings_file` configuration.

`modified_file_path` now points to a file that lives one level higher than the
configuration file.