Fix directory of generated files for PHP library

[nickevansuk]

Fix directory/references for generated files as noted here: openactive/models-php#79

Perhaps the issue is about the references rather than the directories?

[kjlloydie]

Potentially related: openactive/models-php#84 - the filename 3DModel.php is inconsistent with the naming of the ThreeDModel class, resulting in an inability to generate optimized autoload files in composer.

[nickevansuk]

Original issue closed by #55, @kjlloydie I've reraised your comment as a separate issue #56