Fix instructions in README.markdown regarding Rails 3.x reloading endpoint per request

The original instructions didn't work for me on Rails 3.2.11. I used the instructions on the wiki and it works fine (https://github.com/intridea/grape/wiki/Reload-Grape-API-on-every-request-in-Rails).

Edited the README.markdown file to correct this since these instructions caused me to waste time trying to figure out how to get reloading working.