Update intro_inventory.rst

New to ansible, but installing via HomeBrew on Mac OS X (Mountain Lion), I discovered from a note on http://lampros.chaidas.com/index.php/2014/06/22/beginning-with-ansible-on-freebsd-10/ that the inventory file (`hosts`) needs to be in `/usr/local/etc/ansible` not `/etc/ansible`. Submitted change for Intro doc as well. With a hosts file in `/etc/ansible`, ansible did not detect it, and continued to prompt me to specify one with the "-i" parameter. Am I wrong, or is this an OS X anomaly?