refer to the correct env var NEWRELIC_AGENT_ENABLED

In turns out that the documentation in https://docs.newrelic.com/docs/ruby/forcing-the-ruby-agent-to-start and the logs are referring to the non-working environment variable `NEWRELIC_ENABLE`. However, from my own testing and from "lib/new_relic/control/instance_methods.rb" line 76, it seems that the correct var is `NEWRELIC_AGENT_ENABLED`.