docs: fix env-cmd example in deployment section

[VMois]

Related issue #6995

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[VMois]

env-cmd have changed a little bit. Reference to the current working example - https://github.com/toddbluhm/env-cmd-examples/blob/master/file-example/package.json . Based on this example, I have updated docs.

[iansu]

Thanks. This looks good but you have to sign the CLA before we can accept it.

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[iansu]

Thanks!