Update configuration instructions for Apache 2.4

Summary:
Fixes T4061. Following the instructions in the documentation with Apache 2.4 (which is installed
with Ubuntu 13.10 and other distributions) will result in a "403 forbidden" error.

The instruction provides information on how to fix it.

Test Plan: Tested on apache 2.4 install

Reviewers: epriestley, Blessed Reviewers

Reviewed By: epriestley

CC: Korvin, epriestley, aran, asherkin

Maniphest Tasks: T4061

Differential Revision: https://secure.phabricator.com/D7529