Luckily this is an easy fix. You error message will more accurately say something along the lines of:
A server is already running. Check /home/greg/odin/spitter/tmp/pids/server.pid.
If you run
cat on the file referenced in the message, it will return a 5 digit number like 30351.
30351 is the ID of the process running the current rails server. Run
kill -9 30351 to end the process and free up room for a new server.
You are now free to run
rails s to boot up the server.