Fix nginx configuration guide

Summary: We currently match "any PHP file path", which is wrong, since it will match things like /diffusion/path/to/some/source/code.php. Match only "index.php".

Test Plan: This is the config secure.phabricator.com / local / etc run, we just had out of date documentation.

Reviewers: Korvin, vrana, btrahan, jungejason

Reviewed By: jungejason

CC: aran

Maniphest Tasks: T1323

Differential Revision: https://secure.phabricator.com/D2754