fixed typo in example

I didn't try to improve the postgres instructions, but I found it really hard to setup. It took a while to get it working and it was hard to find the clues that I needed. Even figuring out the default username and password on the database that I setup was confusing. Some things that helped me were these links:

Starting postgres was not immediately clear.
http://stackoverflow.com/questions/7975556/how-to-start-postgresql-server-on-mac-os-x

I needed the pg gem, but had trouble installing it until I built native extensions.
http://stackoverflow.com/questions/6209797/cant-find-the-postgresql-client-library-libpq