Update instructions for setting up Nginx

It appears that the default Nginx setup has a line for:

    passenger_ruby /usr/bin/ruby;

instead of:

    passenger_ruby /home/deploy/.rbenv/shims/ruby;

This update adjusts the text of the setup instructions to point out that this needs to be uncommented **and** modified. If this change isn't made during setup you will see an error when you restart the Nginx service to start using Passenger.
```
deploy@exchange:~/peatio/current$ sudo service nginx restart
* Restarting nginx nginx                                                [fail]
```
Merge pull request #249 from brossi/patch-3

Update instructions for setting up Nginx