docs(guides): correct bash command in environment-variables.md

[iamspark1e]

Hello, these days I upgrade my project into webpack 5 (currently 5.3.1), and I found a little change in environment variables.
According to the issue #1216 and pull request, --env.foo=bar should be changed to --env foo=bar, so I created this pull request.

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[montogeek]

Thanks!