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Test Plan: This is the config secure.phabricator.com / local / etc run, we just had out of date documentation.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

src/docs/configuration/configuration_guide.diviner

@@ -145,7 +145,7 @@ For nginx, use a configuration like this:
       }
     }
 
-    location ~ \.php$ {
+    location /index.php {
       fastcgi_pass   localhost:9000;
       fastcgi_index   index.php;