Update nginx configuration documentation.

Summary: The configuration guide for nginx recommends using a conditional statement to check if a file exists, which is listed on the nginx wiki as a common pitfall. It is not necessary to use a conditional and probably should not be recommended to do so.

Test Plan: We use this nginx configuration in our installation.

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D9401