Fix env vars table in README

[fiveangle]

ps- you might consider enabling the issue tracker, since no one can alert you of problems other than submitted a PR. Also, there seems to be other forks with worthwhile changes. Your shutdown feature is the most useful addition but you might consider checking out these others to see if they might benefit yours at all 🤷‍♂️ : https://github.com/DualBit/go-rest-wol/network

Here's some of the more interesting ones:

• store csv in a separately mapped docker volume (esp useful for portainer/docker-compose implementations - I've done this on my own build but this would be a very nice addition): https://github.com/jlittl80/rpi-go-rest-wol/
• "ping" api added (haven't tried this one yet as there were merge conflicts with yours): https://github.com/alfonsocatanzaro/go-rest-wol
• support for running on rpi: https://github.com/nagubal/go-rest-wol/

Thanks for adding the shutdown feature !