Fix filenames of installed documentation Since 0.6.0, upstream renamed some of their documentation files in the source tarball. However, OpenTTD expect specific filenames for these after installation, to allow displaying these files in the GUI. For generated tarballs, the upstream Makefile already corrected these filenames, but for directory installs it used the modified filenames as-is, preventing OpenTTD from finding these files.
This patches the Makefile to use the proper filenames again.
