Update dotenv example in env.md

[rossta]

Removes use of EnvironmentPlugin from the dotenv example in the docs.

The base webpack config already adds the EnvironmentPlugin with JSON-ified process.env:

webpacker/package/environments/base.js

Lines 31 to 34 in a84a4bb

	result.append(
	'Environment',
	new webpack.EnvironmentPlugin(JSON.parse(JSON.stringify(process.env)))
	)

[np6176a]

I'm not sure why this was merged. This feature doesn't work with this line removed.

[rromanchuk]

@np6176a are you on 6 or 5? i can't access process.env except for in config. No idea what i'm doing lol